6 MySQL Language Reference

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6 MySQL Language Reference

MySQL has a very complex, but intuitive and easy to learn SQL interface. This chapter describes the various commands, types, and functions you will need to know in order to use MySQL efficiently and effectively. This chapter also serves as a reference to all functionality included in MySQL. In order to use this chapter effectively, you may find it useful to refer to the various indexes.

6.1 Language Structure

6.1.1 Literals: How to Write Strings and Numbers

This section describes the various ways to write strings and numbers in MySQL. It also covers the various nuances and ``gotchas'' that you may run into when dealing with these basic types in MySQL.

6.1.1.1 Strings

A string is a sequence of characters, surrounded by either single quote (`'') or double quote (`"') characters (only the single quote if you run in ANSI mode). Examples: 

'a string'
"another string"

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (`\'), known as the escape character. MySQL recognizes the following escape sequences:

\0
An ASCII 0 (NUL) character.
\'
A single quote (`'') character.
\"
A double quote (`"') character.
\b
A backspace character.
\n
A newline character.
\r
A carriage return character.
\t
A tab character.
\z
ASCII(26) (Control-Z). This character can be encoded to allow you to go around the problem that ASCII(26) stands for END-OF-FILE on Windows. (ASCII(26) will cause problems if you try to use mysql database < filename).
\\
A backslash (`\') character.
\%
A `%' character. This is used to search for literal instances of `%' in contexts where `%' would otherwise be interpreted as a wild-card character. See section 6.3.2.1 String Comparison Functions.
\_
A `_' character. This is used to search for literal instances of `_' in contexts where `_' would otherwise be interpreted as a wild-card character. See section 6.3.2.1 String Comparison Functions.

Note that if you use `\%' or `\_' in some string contexts, these will return the strings `\%' and `\_' and not `%' and `_'.

There are several ways to include quotes within a string:

The SELECT statements shown below demonstrate how quoting and escaping work: 

mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello';
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello";
+-------+---------+-----------+--------+--------+
| hello | 'hello' | ''hello'' | hel"lo | "hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "This\nIs\nFour\nlines";
+--------------------+
| This
Is
Four
lines |
+--------------------+

If you want to insert binary data into a BLOB column, the following characters must be represented by escape sequences:

NUL
ASCII 0. You should represent this by `\0' (a backslash and an ASCII `0' character).
\
ASCII 92, backslash. Represent this by `\\'.
'
ASCII 39, single quote. Represent this by `\''.
"
ASCII 34, double quote. Represent this by `\"'.

If you write C code, you can use the C API function mysql_escape_string() to escape characters for the INSERT statement. See section 8.4.2 C API Function Overview. In Perl, you can use the quote method of the DBI package to convert special characters to the proper escape sequences. See section 8.2.2 The DBI Interface.

You should use an escape function on any string that might contain any of the special characters listed above!

6.1.1.2 Numbers

Integers are represented as a sequence of digits. Floats use `.' as a decimal separator. Either type of number may be preceded by `-' to indicate a negative value.

Examples of valid integers: 

1221
0
-32

Examples of valid floating-point numbers: 

294.42
-32032.6809e+10
148.00

An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.

6.1.1.3 Hexadecimal Values

MySQL supports hexadecimal values. In number context these act like an integer (64-bit precision). In string context these act like a binary string where each pair of hex digits is converted to a character: 

mysql> SELECT x'FF'
       -> 255
mysql> SELECT 0xa+0;
       -> 10
mysql> select 0x5061756c;
       -> Paul

The x'hexstring' syntax (new in 4.0) is based on ANSI SQL and the 0x syntax is based on ODBC. Hexadecimal strings are often used by ODBC to give values for BLOB columns.

6.1.1.4 NULL Values

The NULL value means ``no data'' and is different from values such as 0 for numeric types or the empty string for string types. See section A.5.3 Problems with NULL Values.

NULL may be represented by \N when using the text file import or export formats (LOAD DATA INFILE, SELECT ... INTO OUTFILE). See section 6.4.9 LOAD DATA INFILE Syntax.

6.1.2 Database, Table, Index, Column, and Alias Names

Database, table, index, column, and alias names all follow the same rules in MySQL.

Note that the rules changed starting with MySQL Version 3.23.6 when we introduced quoting of identifiers (database, table, and column names) with ``'. `"' will also work to quote identifiers if you run in ANSI mode. See section 1.4.3 Running MySQL in ANSI Mode.

Identifier Max length Allowed characters
Database 64 Any character that is allowed in a directory name except `/' or `.'.
Table 64 Any character that is allowed in a file name, except `/' or `.'.
Column 64 All characters.
Alias 255 All characters.

Note that in addition to the above, you can't have ASCII(0) or ASCII(255) or the quoting character in an identifier.

Note that if the identifier is a restricted word or contains special characters you must always quote it with ` when you use it: 

SELECT * from `select` where `select`.id > 100;

In previous versions of MySQL, the name rules are as follows:

It is recommended that you do not use names like 1e, because an expression like 1e+1 is ambiguous. It may be interpreted as the expression 1e + 1 or as the number 1e+1.

In MySQL you can refer to a column using any of the following forms:

Column reference Meaning
col_name Column col_name from whichever table used in the query contains a column of that name.
tbl_name.col_name Column col_name from table tbl_name of the current database.
db_name.tbl_name.col_name Column col_name from table tbl_name of the database db_name. This form is available in MySQL Version 3.22 or later.
`column_name` A column that is a keyword or contains special characters.

You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a statement unless the reference would be ambiguous. For example, suppose tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement, so you must indicate which table you mean by writing t1.c or t2.c. Similarly, if you are retrieving from a table t in database db1 and from a table t in database db2, you must refer to columns in those tables as db1.t.col_name and db2.t.col_name.

The syntax .tbl_name means the table tbl_name in the current database. This syntax is accepted for ODBC compatibility, because some ODBC programs prefix table names with a `.' character.

6.1.3 Case Sensitivity in Names

In MySQL, databases and tables correspond to directories and files within those directories. Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are case sensitive in Unix and case insensitive in Windows. See section 1.4.1 MySQL Extensions to ANSI SQL92.

NOTE: Although database and table names are case insensitive for Windows, you should not refer to a given database or table using different cases within the same query. The following query would not work because it refers to a table both as my_table and as MY_TABLE: 

mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column names are case insensitive in all cases.

Aliases on tables are case sensitive. The following query would not work because it refers to the alias both as a and as A: 

mysql> SELECT col_name FROM tbl_name AS a
           WHERE a.col_name = 1 OR A.col_name = 2;

Aliases on columns are case insensitive.

If you have a problem remembering the used cases for a table names, adopt a consistent convention, such as always creating databases and tables using lowercase names.

One way to avoid this problem is to start mysqld with -O lower_case_table_names=1. By default this option is 1 on Windows and 0 on Unix.

If lower_case_table_names is 1 MySQL will convert all table names to lower case on storage and lookup. Note that if you change this option, you need to first convert your old table names to lower case before starting mysqld.

6.1.4 User Variables

MySQL supports thread-specific variables with the @variablename syntax. A variable name may consist of alphanumeric characters from the current character set and also `_', `$', and `.' . The default character set is ISO-8859-1 Latin1; this may be changed with the --default-character-set option to mysqld. See section 4.6.1 The Character Set Used for Data and Sorting.

Variables don't have to be initialized. They contain NULL by default and can store an integer, real, or string value. All variables for a thread are automatically freed when the thread exits.

You can set a variable with the SET syntax: 

SET @variable= { integer expression | real expression | string expression }
[,@variable= ...].

You can also set a variable in an expression with the @variable:=expr syntax: 

select @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
+----------------------+------+------+------+
| @t1:=(@t2:=1)+@t3:=4 | @t1  | @t2  | @t3  |
+----------------------+------+------+------+
|                    5 |    5 |    1 |    4 |
+----------------------+------+------+------+

(We had to use the := syntax here, because = was reserved for comparisons.)

User variables may be used where expressions are allowed. Note that this does not currently include use in contexts where a number is explicitly required, such as in the LIMIT clause of a SELECT statement, or the IGNORE number LINES clause of a LOAD DATA statement.

NOTE: In a SELECT statement, each expression is only evaluated when it's sent to the client. This means that in the HAVING, GROUP BY, or ORDER BY clause, you can't refer to an expression that involves variables that are set in the SELECT part. For example, the following statement will NOT work as expected: 

SELECT (@aa:=id) AS a, (@aa+3) AS b FROM table_name HAVING b=5;

The reason is that @aa will not contain the value of the current row, but the value of id for the previous accepted row.

6.1.5 Comment Syntax

The MySQL server supports the # to end of line, -- to end of line and /* in-line or multiple-line */ comment styles: 

mysql> select 1+1;     # This comment continues to the end of line
mysql> select 1+1;     -- This comment continues to the end of line
mysql> select 1 /* this is an in-line comment */ + 1;
mysql> select 1+
/*
this is a
multiple-line comment
*/
1;

Note that the -- comment style requires you to have at least one space after the --!

Although the server understands the comment syntax just described, there are some limitations on the way that the mysql client parses /* ... */ comments:

These limitations apply both when you run mysql interactively and when you put commands in a file and tell mysql to read its input from that file with mysql < some-file.

MySQL doesn't support the `--' ANSI SQL comment style. See section 1.4.4.8 `--' as the Start of a Comment.

6.1.6 Is MySQL Picky About Reserved Words?

A common problem stems from trying to create a table with column names that use the names of datatypes or functions built into MySQL, such as TIMESTAMP or GROUP. You're allowed to do it (for example, ABS is an allowed column name), but whitespace is not allowed between a function name and the `(' when using functions whose names are also column names.

The following words are explicitly reserved in MySQL. Most of them are forbidden by ANSI SQL92 as column and/or table names (for example, group). A few are reserved because MySQL needs them and is (currently) using a yacc parser:

action add aggregate all
alter after and as
asc avg avg_row_length auto_increment
between bigint bit binary
blob bool both by
cascade case char character
change check checksum column
columns comment constraint create
cross current_date current_time current_timestamp
data database databases date
datetime day day_hour day_minute
day_second dayofmonth dayofweek dayofyear
dec decimal default delayed
delay_key_write delete desc describe
distinct distinctrow double drop
end else escape escaped
enclosed enum explain exists
fields file first float
float4 float8 flush foreign
from for full function
global grant grants group
having heap high_priority hour
hour_minute hour_second hosts identified
ignore in index infile
inner insert insert_id int
integer interval int1 int2
int3 int4 int8 into
if is isam join
key keys kill last_insert_id
leading left length like
lines limit load local
lock logs long longblob
longtext low_priority max max_rows
match mediumblob mediumtext mediumint
middleint min_rows minute minute_second
modify month monthname myisam
natural numeric no not
null on optimize option
optionally or order outer
outfile pack_keys partial password
precision primary procedure process
processlist privileges read real
references reload regexp rename
replace restrict returns revoke
rlike row rows second
select set show shutdown
smallint soname sql_big_tables sql_big_selects
sql_low_priority_updates sql_log_off sql_log_update sql_select_limit
sql_small_result sql_big_result sql_warnings straight_join
starting status string table
tables temporary terminated text
then time timestamp tinyblob
tinytext tinyint trailing to
type use using unique
unlock unsigned update usage
values varchar variables varying
varbinary with write when
where year year_month zerofill

The following symbols (from the table above) are disallowed by ANSI SQL but allowed by MySQL as column/table names. This is because some of these names are very natural names and a lot of people have already used them.

6.2 Column Types

MySQL supports a number of column types, which may be grouped into three categories: numeric types, date and time types, and string (character) types. This section first gives an overview of the types available and summarizes the storage requirements for each column type, then provides a more detailed description of the properties of the types in each category. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values.

The column types supported by MySQL are listed below. The following code letters are used in the descriptions:

M
Indicates the maximum display size. The maximum legal display size is 255.
D
Applies to floating-point types and indicates the number of digits following the decimal point. The maximum possible value is 30, but should be no greater than M-2.

Square brackets (`[' and `]') indicate parts of type specifiers that are optional.

Note that if you specify ZEROFILL for a column, MySQL will automatically add the UNSIGNED attribute to the column.

TINYINT[(M)] [UNSIGNED] [ZEROFILL]
A very small integer. The signed range is -128 to 127. The unsigned range is 0 to 255.
SMALLINT[(M)] [UNSIGNED] [ZEROFILL]
A small integer. The signed range is -32768 to 32767. The unsigned range is 0 to 65535.
MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]
A medium-size integer. The signed range is -8388608 to 8388607. The unsigned range is 0 to 16777215.
INT[(M)] [UNSIGNED] [ZEROFILL]
A normal-size integer. The signed range is -2147483648 to 2147483647. The unsigned range is 0 to 4294967295.
INTEGER[(M)] [UNSIGNED] [ZEROFILL]
This is a synonym for INT.
BIGINT[(M)] [UNSIGNED] [ZEROFILL]
A large integer. The signed range is -9223372036854775808 to 9223372036854775807. The unsigned range is 0 to 18446744073709551615. Some things you should be aware about BIGINT columns:
FLOAT(precision) [ZEROFILL]
A floating-point number. Cannot be unsigned. precision can be <=24 for a single-precision floating-point number and between 25 and 53 for a double-precision floating-point number. These types are like the FLOAT and DOUBLE types described immediately below. FLOAT(X) has the same range as the corresponding FLOAT and DOUBLE types, but the display size and number of decimals is undefined. In MySQL Version 3.23, this is a true floating-point value. In earlier MySQL versions, FLOAT(precision) always has 2 decimals. Note that using FLOAT may give you some unexpected problems as all calculation in MySQL is done with double precision. See section A.5.6 Solving Problems with No Matching Rows. This syntax is provided for ODBC compatibility.
FLOAT[(M,D)] [ZEROFILL]
A small (single-precision) floating-point number. Cannot be unsigned. Allowable values are -3.402823466E+38 to -1.175494351E-38, 0, and 1.175494351E-38 to 3.402823466E+38. The M is the display width and D is the number of decimals. FLOAT without an argument or with an argument of <= 24 stands for a single-precision floating-point number.
DOUBLE[(M,D)] [ZEROFILL]
A normal-size (double-precision) floating-point number. Cannot be unsigned. Allowable values are -1.7976931348623157E+308 to -2.2250738585072014E-308, 0, and 2.2250738585072014E-308 to 1.7976931348623157E+308. The M is the display width and D is the number of decimals. DOUBLE without an argument or FLOAT(X) where 25 <= X <= 53 stands for a double-precision floating-point number.
DOUBLE PRECISION[(M,D)] [ZEROFILL]
REAL[(M,D)] [ZEROFILL]
These are synonyms for DOUBLE.
DECIMAL[(M[,D])] [ZEROFILL]
An unpacked floating-point number. Cannot be unsigned. Behaves like a CHAR column: ``unpacked'' means the number is stored as a string, using one character for each digit of the value. The decimal point and, for negative numbers, the `-' sign, are not counted in M (but space for these are reserved). If D is 0, values will have no decimal point or fractional part. The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column may be constrained by the choice of M and D. If D is left out it's set to 0. If M is left out it's set to 10. Note that in MySQL Version 3.22 the M argument had to includes the space needed for the sign and the decimal point.
NUMERIC(M,D) [ZEROFILL]
This is a synonym for DECIMAL.
DATE
A date. The supported range is '1000-01-01' to '9999-12-31'. MySQL displays DATE values in 'YYYY-MM-DD' format, but allows you to assign values to DATE columns using either strings or numbers. See section 6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types.
DATETIME
A date and time combination. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. MySQL displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format, but allows you to assign values to DATETIME columns using either strings or numbers. See section 6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types.
TIMESTAMP[(M)]
A timestamp. The range is '1970-01-01 00:00:00' to sometime in the year 2037. MySQL displays TIMESTAMP values in YYYYMMDDHHMMSS, YYMMDDHHMMSS, YYYYMMDD, or YYMMDD format, depending on whether M is 14 (or missing), 12, 8, or 6, but allows you to assign values to TIMESTAMP columns using either strings or numbers. A TIMESTAMP column is useful for recording the date and time of an INSERT or UPDATE operation because it is automatically set to the date and time of the most recent operation if you don't give it a value yourself. You can also set it to the current date and time by assigning it a NULL value. See section 6.2.2 Date and Time Types. A TIMESTAMP is always stored in 4 bytes. The M argument only affects how the TIMESTAMP column is displayed. Note that TIMESTAMP(X) columns where X is 8 or 14 are reported to be numbers while other TIMESTAMP(X) columns are reported to be strings. This is just to ensure that one can reliably dump and restore the table with these types! See section 6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types.
TIME
A time. The range is '-838:59:59' to '838:59:59'. MySQL displays TIME values in 'HH:MM:SS' format, but allows you to assign values to TIME columns using either strings or numbers. See section 6.2.2.3 The TIME Type.
YEAR[(2|4)]
A year in 2- or 4-digit format (default is 4-digit). The allowable values are 1901 to 2155, 0000 in the 4-digit year format, and 1970-2069 if you use the 2-digit format (70-69). MySQL displays YEAR values in YYYY format, but allows you to assign values to YEAR columns using either strings or numbers. (The YEAR type is new in MySQL Version 3.22.). See section 6.2.2.4 The YEAR Type.
[NATIONAL] CHAR(M) [BINARY]
A fixed-length string that is always right-padded with spaces to the specified length when stored. The range of M is 1 to 255 characters. Trailing spaces are removed when the value is retrieved. CHAR values are sorted and compared in case-insensitive fashion according to the default character set unless the BINARY keyword is given. NATIONAL CHAR (short form NCHAR) is the ANSI SQL way to define that a CHAR column should use the default CHARACTER set. This is the default in MySQL. CHAR is a shorthand for CHARACTER. MySQL allows you to create a column of type CHAR(0). This is mainly useful when you have to be compliant with some old applications that depend on the existence of a column but that do not actually use the value. This is also quite nice when you need a column that only can take 2 values: A CHAR(0), that is not defined as NOT NULL, will only occupy one bit and can only take 2 values: NULL or "". See section 6.2.3.1 The CHAR and VARCHAR Types.
[NATIONAL] VARCHAR(M) [BINARY]
A variable-length string. NOTE: Trailing spaces are removed when the value is stored (this differs from the ANSI SQL specification). The range of M is 1 to 255 characters. VARCHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given. See section 6.5.3.1 Silent Column Specification Changes. VARCHAR is a shorthand for CHARACTER VARYING. See section 6.2.3.1 The CHAR and VARCHAR Types.
TINYBLOB
TINYTEXT
A BLOB or TEXT column with a maximum length of 255 (2^8 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The BLOB and TEXT Types.
BLOB
TEXT
A BLOB or TEXT column with a maximum length of 65535 (2^16 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The BLOB and TEXT Types.
MEDIUMBLOB
MEDIUMTEXT
A BLOB or TEXT column with a maximum length of 16777215 (2^24 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The BLOB and TEXT Types.
LONGBLOB
LONGTEXT
A BLOB or TEXT column with a maximum length of 4294967295 (2^32 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. Note that because the server/client protocol and MyISAM tables has currently a limit of 16M per communication packet / table row, you can't yet use this the whole range of this type. See section 6.2.3.2 The BLOB and TEXT Types.
ENUM('value1','value2',...)
An enumeration. A string object that can have only one value, chosen from the list of values 'value1', 'value2', ..., NULL or the special "" error value. An ENUM can have a maximum of 65535 distinct values. See section 6.2.3.3 The ENUM Type.
SET('value1','value2',...)
A set. A string object that can have zero or more values, each of which must be chosen from the list of values 'value1', 'value2', ... A SET can have a maximum of 64 members. See section 6.2.3.4 The SET Type.

6.2.1 Numeric Types

MySQL supports all of the ANSI/ISO SQL92 numeric types. These types include the exact numeric data types (NUMERIC, DECIMAL, INTEGER, and SMALLINT), as well as the approximate numeric data types (FLOAT, REAL, and DOUBLE PRECISION). The keyword INT is a synonym for INTEGER, and the keyword DEC is a synonym for DECIMAL.

The NUMERIC and DECIMAL types are implemented as the same type by MySQL, as permitted by the SQL92 standard. They are used for values for which it is important to preserve exact precision, for example with monetary data. When declaring a column of one of these types the precision and scale can be (and usually is) specified; for example: 

    salary DECIMAL(9,2)

In this example, 9 (precision) represents the number of significant decimal digits that will be stored for values, and 2 (scale) represents the number of digits that will be stored following the decimal point. In this case, therefore, the range of values that can be stored in the salary column is from -9999999.99 to 9999999.99. (MySQL can actually store numbers up to 9999999.99 in this column because it doesn't have to store the sign for positive numbers)

In ANSI/ISO SQL92, the syntax DECIMAL(p) is equivalent to DECIMAL(p,0). Similarly, the syntax DECIMAL is equivalent to DECIMAL(p,0), where the implementation is allowed to decide the value of p. MySQL does not currently support either of these variant forms of the DECIMAL/NUMERIC data types. This is not generally a serious problem, as the principal benefits of these types derive from the ability to control both precision and scale explicitly.

DECIMAL and NUMERIC values are stored as strings, rather than as binary floating-point numbers, in order to preserve the decimal precision of those values. One character is used for each digit of the value, the decimal point (if scale > 0), and the `-' sign (for negative numbers). If scale is 0, DECIMAL and NUMERIC values contain no decimal point or fractional part.

The maximum range of DECIMAL and NUMERIC values is the same as for DOUBLE, but the actual range for a given DECIMAL or NUMERIC column can be constrained by the precision or scale for a given column. When such a column is assigned a value with more digits following the decimal point than are allowed by the specified scale, the value is rounded to that scale. When a DECIMAL or NUMERIC column is assigned a value whose magnitude exceeds the range implied by the specified (or defaulted) precision and scale, MySQL stores the value representing the corresponding end point of that range.

As an extension to the ANSI/ISO SQL92 standard, MySQL also supports the integral types TINYINT, MEDIUMINT, and BIGINT as listed in the tables above. Another extension is supported by MySQL for optionally specifying the display width of an integral value in parentheses following the base keyword for the type (for example, INT(4)). This optional width specification is used to left-pad the display of values whose width is less than the width specified for the column, but does not constrain the range of values that can be stored in the column, nor the number of digits that will be displayed for values whose width exceeds that specified for the column. When used in conjunction with the optional extension attribute ZEROFILL, the default padding of spaces is replaced with zeroes. For example, for a column declared as INT(5) ZEROFILL, a value of 4 is retrieved as 00004. Note that if you store larger values than the display width in an integer column, you may experience problems when MySQL generates temporary tables for some complicated joins, as in these cases MySQL trusts that the data did fit into the original column width.

All integral types can have an optional (non-standard) attribute UNSIGNED. Unsigned values can be used when you want to allow only positive numbers in a column and you need a little bigger numeric range for the column.

The FLOAT type is used to represent approximate numeric data types. The ANSI/ISO SQL92 standard allows an optional specification of the precision (but not the range of the exponent) in bits following the keyword FLOAT in parentheses. The MySQL implementation also supports this optional precision specification. When the keyword FLOAT is used for a column type without a precision specification, MySQL uses four bytes to store the values. A variant syntax is also supported, with two numbers given in parentheses following the FLOAT keyword. With this option, the first number continues to represent the storage requirements for the value in bytes, and the second number specifies the number of digits to be stored and displayed following the decimal point (as with DECIMAL and NUMERIC). When MySQL is asked to store a number for such a column with more decimal digits following the decimal point than specified for the column, the value is rounded to eliminate the extra digits when the value is stored.

The REAL and DOUBLE PRECISION types do not accept precision specifications. As an extension to the ANSI/ISO SQL92 standard, MySQL recognizes DOUBLE as a synonym for the DOUBLE PRECISION type. In contrast with the standard's requirement that the precision for REAL be smaller than that used for DOUBLE PRECISION, MySQL implements both as 8-byte double-precision floating-point values (when not running in ``ANSI mode''). For maximum portability, code requiring storage of approximate numeric data values should use FLOAT or DOUBLE PRECISION with no specification of precision or number of decimal points.

When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead.

For example, the range of an INT column is -2147483648 to 2147483647. If you try to insert -9999999999 into an INT column, the value is clipped to the lower endpoint of the range, and -2147483648 is stored instead. Similarly, if you try to insert 9999999999, 2147483647 is stored instead.

If the INT column is UNSIGNED, the size of the column's range is the same but its endpoints shift up to 0 and 4294967295. If you try to store -9999999999 and 9999999999, the values stored in the column become 0 and 4294967296.

Conversions that occur due to clipping are reported as ``warnings'' for ALTER TABLE, LOAD DATA INFILE, UPDATE, and multi-row INSERT statements.

6.2.2 Date and Time Types

The date and time types are DATETIME, DATE, TIMESTAMP, TIME, and YEAR. Each of these has a range of legal values, as well as a ``zero'' value that is used when you specify a really illegal value. Note that MySQL allows you to store certain 'not strictly' legal date values, for example 1999-11-31. The reason for this is that we think it's the responsibility of the application to handle date checking, not the SQL servers. To make the date checking 'fast', MySQL only checks that the month is in the range of 0-12 and the day is in the range of 0-31. The above ranges are defined this way because MySQL allows you to store, in a DATE or DATETIME column, dates where the day or month-day is zero. This is extremely useful for applications that need to store a birth-date for which you don't know the exact date. In this case you simply store the date like 1999-00-00 or 1999-01-00. (You cannot expect to get a correct value from functions like DATE_SUB() or DATE_ADD for dates like these.)

Here are some general considerations to keep in mind when working with date and time types:

6.2.2.1 Y2K Issues and Date Types

MySQL itself is Y2K-safe (see section 1.1.9 Year 2000 Compliance), but input values presented to MySQL may not be. Any input containing 2-digit year values is ambiguous, because the century is unknown. Such values must be interpreted into 4-digit form because MySQL stores years internally using four digits.

For DATETIME, DATE, TIMESTAMP, and YEAR types, MySQL interprets dates with ambiguous year values using the following rules:

Remember that these rules provide only reasonable guesses as to what your data mean. If the heuristics used by MySQL don't produce the correct values, you should provide unambiguous input containing 4-digit year values.

ORDER BY will sort 2-digit YEAR/DATE/DATETIME types properly.

Note also that some functions like MIN() and MAX() will convert a TIMESTAMP/DATE to a number. This means that a timestamp with a 2-digit year will not work properly with these functions. The fix in this case is to convert the TIMESTAMP/DATE to 4-digit year format or use something like MIN(DATE_ADD(timestamp,INTERVAL 0 DAYS)).

6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types

The DATETIME, DATE, and TIMESTAMP types are related. This section describes their characteristics, how they are similar, and how they differ.

The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. (``Supported'' means that although earlier values might work, there is no guarantee that they will.)

The DATE type is used when you need only a date value, without a time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.

The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns, only the first one is updated automatically.

Automatic updating of the first TIMESTAMP column occurs under any of the following conditions:

TIMESTAMP columns other than the first may also be set to the current date and time. Just set the column to NULL or to NOW().

You can set any TIMESTAMP column to a value different than the current date and time by setting it explicitly to the desired value. This is true even for the first TIMESTAMP column. You can use this property if, for example, you want a TIMESTAMP to be set to the current date and time when you create a row, but not to be changed whenever the row is updated later:

On the other hand, you may find it just as easy to use a DATETIME column that you initialize to NOW() when the row is created and leave alone for subsequent updates.

TIMESTAMP values may range from the beginning of 1970 to sometime in the year 2037, with a resolution of one second. Values are displayed as numbers.

The format in which MySQL retrieves and displays TIMESTAMP values depends on the display size, as illustrated by the table below. The `full' TIMESTAMP format is 14 digits, but TIMESTAMP columns may be created with shorter display sizes:

Column type Display format
TIMESTAMP(14) YYYYMMDDHHMMSS
TIMESTAMP(12) YYMMDDHHMMSS
TIMESTAMP(10) YYMMDDHHMM
TIMESTAMP(8) YYYYMMDD
TIMESTAMP(6) YYMMDD
TIMESTAMP(4) YYMM
TIMESTAMP(2) YY

All TIMESTAMP columns have the same storage size, regardless of display size. The most common display sizes are 6, 8, 12, and 14. You can specify an arbitrary display size at table creation time, but values of 0 or greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.

You can specify DATETIME, DATE, and TIMESTAMP values using any of a common set of formats:

Illegal DATETIME, DATE, or TIMESTAMP values are converted to the ``zero'' value of the appropriate type ('0000-00-00 00:00:00', '0000-00-00', or 00000000000000).

For values specified as strings that include date part delimiters, it is not necessary to specify two digits for month or day values that are less than 10. '1979-6-9' is the same as '1979-06-09'. Similarly, for values specified as strings that include time part delimiters, it is not necessary to specify two digits for hour, month, or second values that are less than 10. '1979-10-30 1:2:3' is the same as '1979-10-30 01:02:03'.

Values specified as numbers should be 6, 8, 12, or 14 digits long. If the number is 8 or 14 digits long, it is assumed to be in YYYYMMDD or YYYYMMDDHHMMSS format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in YYMMDD or YYMMDDHHMMSS format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length.

Values specified as non-delimited strings are interpreted using their length as given. If the string is 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute, and second values, for as many parts as are present in the string. This means you should not use strings that have fewer than 6 characters. For example, if you specify '9903', thinking that will represent March, 1999, you will find that MySQL inserts a ``zero'' date into your table. This is because the year and month values are 99 and 03, but the day part is missing (zero), so the value is not a legal date.

TIMESTAMP columns store legal values using the full precision with which the value was specified, regardless of the display size. This has several implications:

You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:

Be aware of certain pitfalls when specifying date values:

6.2.2.3 The TIME Type

MySQL retrieves and displays TIME values in 'HH:MM:SS' format (or 'HHH:MM:SS' format for large hours values). TIME values may range from '-838:59:59' to '838:59:59'. The reason the hours part may be so large is that the TIME type may be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative).

You can specify TIME values in a variety of formats:

For TIME values specified as strings that include a time part delimiter, it is not necessary to specify two digits for hours, minutes, or seconds values that are less than 10. '8:3:2' is the same as '08:03:02'.

Be careful about assigning ``short'' TIME values to a TIME column. Without semicolon, MySQL interprets values using the assumption that the rightmost digits represent seconds. (MySQL interprets TIME values as elapsed time rather than as time of day.) For example, you might think of '1112' and 1112 as meaning '11:12:00' (12 minutes after 11 o'clock), but MySQL interprets them as '00:11:12' (11 minutes, 12 seconds). Similarly, '12' and 12 are interpreted as '00:00:12'. TIME values with semicolon, instead, are always treated as time of the day. That is '11:12' will mean '11:12:00', not '00:11:12'.

Values that lie outside the TIME range but are otherwise legal are clipped to the appropriate endpoint of the range. For example, '-850:00:00' and '850:00:00' are converted to '-838:59:59' and '838:59:59'.

Illegal TIME values are converted to '00:00:00'. Note that because '00:00:00' is itself a legal TIME value, there is no way to tell, from a value of '00:00:00' stored in a table, whether the original value was specified as '00:00:00' or whether it was illegal.

6.2.2.4 The YEAR Type

The YEAR type is a 1-byte type used for representing years.

MySQL retrieves and displays YEAR values in YYYY format. The range is 1901 to 2155.

You can specify YEAR values in a variety of formats:

Illegal YEAR values are converted to 0000.

6.2.3 String Types

The string types are CHAR, VARCHAR, BLOB, TEXT, ENUM, and SET. This section describes how these types work, their storage requirements, and how to use them in your queries.

6.2.3.1 The CHAR and VARCHAR Types

The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved.

The length of a CHAR column is fixed to the length that you declare when you create the table. The length can be any value between 1 and 255. (As of MySQL Version 3.23, the length of CHAR may be 0 to 255.) When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.

Values in VARCHAR columns are variable-length strings. You can declare a VARCHAR column to be any length between 1 and 255, just as for CHAR columns. However, in contrast to CHAR, VARCHAR values are stored using only as many characters as are needed, plus one byte to record the length. Values are not padded; instead, trailing spaces are removed when values are stored. (This space removal differs from the ANSI SQL specification.)

If you assign a value to a CHAR or VARCHAR column that exceeds the column's maximum length, the value is truncated to fit.

The table below illustrates the differences between the two types of columns by showing the result of storing various string values into CHAR(4) and VARCHAR(4) columns:

Value CHAR(4) Storage required VARCHAR(4) Storage required
'' ' ' 4 bytes '' 1 byte
'ab' 'ab ' 4 bytes 'ab' 3 bytes
'abcd' 'abcd' 4 bytes 'abcd' 5 bytes
'abcdefgh' 'abcd' 4 bytes 'abcd' 5 bytes

The values retrieved from the CHAR(4) and VARCHAR(4) columns will be the same in each case, because trailing spaces are removed from CHAR columns upon retrieval.

Values in CHAR and VARCHAR columns are sorted and compared in case-insensitive fashion, unless the BINARY attribute was specified when the table was created. The BINARY attribute means that column values are sorted and compared in case-sensitive fashion according to the ASCII order of the machine where the MySQL server is running. BINARY doesn't affect how the column is stored or retrieved.

The BINARY attribute is sticky. This means that if a column marked BINARY is used in an expression, the whole expression is compared as a BINARY value.

MySQL may silently change the type of a CHAR or VARCHAR column at table creation time. See section 6.5.3.1 Silent Column Specification Changes.

6.2.3.2 The BLOB and TEXT Types

A BLOB is a binary large object that can hold a variable amount of data. The four BLOB types TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB differ only in the maximum length of the values they can hold. See section 6.2.6 Column Type Storage Requirements.

The four TEXT types TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT correspond to the four BLOB types and have the same maximum lengths and storage requirements. The only difference between BLOB and TEXT types is that sorting and comparison is performed in case-sensitive fashion for BLOB values and case-insensitive fashion for TEXT values. In other words, a TEXT is a case-insensitive BLOB.

If you assign a value to a BLOB or TEXT column that exceeds the column type's maximum length, the value is truncated to fit.

In most respects, you can regard a TEXT column as a VARCHAR column that can be as big as you like. Similarly, you can regard a BLOB column as a VARCHAR BINARY column. The differences are:

MyODBC defines BLOB values as LONGVARBINARY and TEXT values as LONGVARCHAR.

Because BLOB and TEXT values may be extremely long, you may run up against some constraints when using them:

Note that each BLOB or TEXT value is represented internally by a separately allocated object. This is in contrast to all other column types, for which storage is allocated once per column when the table is opened.

6.2.3.3 The ENUM Type

An ENUM is a string object whose value normally is chosen from a list of allowed values that are enumerated explicitly in the column specification at table creation time.

The value may also be the empty string ("") or NULL under certain circumstances:

Each enumeration value has an index:

For example, a column specified as ENUM("one", "two", "three") can have any of the values shown below. The index of each value is also shown:

Value Index
NULL NULL
"" 0
"one" 1
"two" 2
"three" 3

An enumeration can have a maximum of 65535 elements.

Lettercase is irrelevant when you assign values to an ENUM column. However, values retrieved from the column later have lettercase matching the values that were used to specify the allowable values at table creation time.

If you retrieve an ENUM in a numeric context, the column value's index is returned. For example, you can retrieve numeric values from an ENUM column like this: 

mysql> SELECT enum_col+0 FROM tbl_name;

If you store a number into an ENUM, the number is treated as an index, and the value stored is the enumeration member with that index. (However, this will not work with LOAD DATA, which treats all input as strings.)

ENUM values are sorted according to the order in which the enumeration members were listed in the column specification. (In other words, ENUM values are sorted according to their index numbers.) For example, "a" sorts before "b" for ENUM("a", "b"), but "b" sorts before "a" for ENUM("b", "a"). The empty string sorts before non-empty strings, and NULL values sort before all other enumeration values.

If you want to get all possible values for an ENUM column, you should use: SHOW COLUMNS FROM table_name LIKE enum_column_name and parse the ENUM definition in the second column.

6.2.3.4 The SET Type

A SET is a string object that can have zero or more values, each of which must be chosen from a list of allowed values specified when the table is created. SET column values that consist of multiple set members are specified with members separated by commas (`,'). A consequence of this is that SET member values cannot themselves contain commas.

For example, a column specified as SET("one", "two") NOT NULL can have any of these values: 

""
"one"
"two"
"one,two"

A SET can have a maximum of 64 different members.

MySQL stores SET values numerically, with the low-order bit of the stored value corresponding to the first set member. If you retrieve a SET value in a numeric context, the value retrieved has bits set corresponding to the set members that make up the column value. For example, you can retrieve numeric values from a SET column like this: 

mysql> SELECT set_col+0 FROM tbl_name;

If a number is stored into a SET column, the bits that are set in the binary representation of the number determine the set members in the column value. Suppose a column is specified as SET("a","b","c","d"). Then the members have the following bit values:

SET member Decimal value Binary value
a 1 0001
b 2 0010
c 4 0100
d 8 1000

If you assign a value of 9 to this column, that is 1001 in binary, so the first and fourth SET value members "a" and "d" are selected and the resulting value is "a,d".

For a value containing more than one SET element, it does not matter what order the elements are listed in when you insert the value. It also does not matter how many times a given element is listed in the value. When the value is retrieved later, each element in the value will appear once, with elements listed according to the order in which they were specified at table creation time. For example, if a column is specified as SET("a","b","c","d"), then "a,d", "d,a", and "d,a,a,d,d" will all appear as "a,d" when retrieved.

SET values are sorted numerically. NULL values sort before non-NULL SET values.

Normally, you perform a SELECT on a SET column using the LIKE operator or the FIND_IN_SET() function: 

mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%';
mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;

But the following will also work: 

mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2';
mysql> SELECT * FROM tbl_name WHERE set_col & 1;

The first of these statements looks for an exact match. The second looks for values containing the first set member.

If you want to get all possible values for a SET column, you should use: SHOW COLUMNS FROM table_name LIKE set_column_name and parse the SET definition in the second column.

6.2.4 Choosing the Right Type for a Column

For the most efficient use of storage, try to use the most precise type in all cases. For example, if an integer column will be used for values in the range between 1 and 99999, MEDIUMINT UNSIGNED is the best type.

Accurate representation of monetary values is a common problem. In MySQL, you should use the DECIMAL type. This is stored as a string, so no loss of accuracy should occur. If accuracy is not too important, the DOUBLE type may also be good enough.

For high precision, you can always convert to a fixed-point type stored in a BIGINT. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary.

6.2.5 Using Column Types from Other Database Engines

To make it easier to use code written for SQL implementations from other vendors, MySQL maps column types as shown in the table below. These mappings make it easier to move table definitions from other database engines to MySQL:

Other vendor type MySQL type
BINARY(NUM) CHAR(NUM) BINARY
CHAR VARYING(NUM) VARCHAR(NUM)
FLOAT4 FLOAT
FLOAT8 DOUBLE
INT1 TINYINT
INT2 SMALLINT
INT3 MEDIUMINT
INT4 INT
INT8 BIGINT
LONG VARBINARY MEDIUMBLOB
LONG VARCHAR MEDIUMTEXT
MIDDLEINT MEDIUMINT
VARBINARY(NUM) VARCHAR(NUM) BINARY

Column type mapping occurs at table creation time. If you create a table with types used by other vendors and then issue a DESCRIBE tbl_name statement, MySQL reports the table structure using the equivalent MySQL types.

6.2.6 Column Type Storage Requirements

The storage requirements for each of the column types supported by MySQL are listed below by category.

6.2.6.1 Storage requirements for numeric types

Column type Storage required
TINYINT 1 byte
SMALLINT 2 bytes
MEDIUMINT 3 bytes
INT 4 bytes
INTEGER 4 bytes
BIGINT 8 bytes
FLOAT(X) 4 if X <= 24 or 8 if 25 <= X <= 53
FLOAT 4 bytes
DOUBLE 8 bytes
DOUBLE PRECISION 8 bytes
REAL 8 bytes
DECIMAL(M,D) M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)
NUMERIC(M,D) M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)

6.2.6.2 Storage requirements for date and time types

Column type Storage required
DATE 3 bytes
DATETIME 8 bytes
TIMESTAMP 4 bytes
TIME 3 bytes
YEAR 1 byte

6.2.6.3 Storage requirements for string types

Column type Storage required
CHAR(M) M bytes, 1 <= M <= 255
VARCHAR(M) L+1 bytes, where L <= M and 1 <= M <= 255
TINYBLOB, TINYTEXT L+1 bytes, where L < 2^8
BLOB, TEXT L+2 bytes, where L < 2^16
MEDIUMBLOB, MEDIUMTEXT L+3 bytes, where L < 2^24
LONGBLOB, LONGTEXT L+4 bytes, where L < 2^32
ENUM('value1','value2',...) 1 or 2 bytes, depending on the number of enumeration values (65535 values maximum)
SET('value1','value2',...) 1, 2, 3, 4 or 8 bytes, depending on the number of set members (64 members maximum)

VARCHAR and the BLOB and TEXT types are variable-length types, for which the storage requirements depend on the actual length of column values (represented by L in the preceding table), rather than on the type's maximum possible size. For example, a VARCHAR(10) column can hold a string with a maximum length of 10 characters. The actual storage required is the length of the string (L), plus 1 byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is 5 bytes.

The BLOB and TEXT types require 1, 2, 3, or 4 bytes to record the length of the column value, depending on the maximum possible length of the type. See section 6.2.3.2 The BLOB and TEXT Types.

If a table includes any variable-length column types, the record format will also be variable-length. Note that when a table is created, MySQL may, under certain conditions, change a column from a variable-length type to a fixed-length type, or vice-versa. See section 6.5.3.1 Silent Column Specification Changes.

The size of an ENUM object is determined by the number of different enumeration values. One byte is used for enumerations with up to 255 possible values. Two bytes are used for enumerations with up to 65535 values. See section 6.2.3.3 The ENUM Type.

The size of a SET object is determined by the number of different set members. If the set size is N, the object occupies (N+7)/8 bytes, rounded up to 1, 2, 3, 4, or 8 bytes. A SET can have a maximum of 64 members. See section 6.2.3.4 The SET Type.

6.3 Functions for Use in SELECT and WHERE Clauses

A select_expression or where_definition in a SQL statement can consist of any expression using the functions described below.

An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression.

NOTE: There must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. Spaces around arguments are permitted, though.

You can force MySQL to accept spaces after the function name by starting mysqld with --ansi or using the CLIENT_IGNORE_SPACE to mysql_connect(), but in this case all function names will become reserved words. See section 1.4.3 Running MySQL in ANSI Mode.

For the sake of brevity, examples display the output from the mysql program in abbreviated form. So this: 

mysql> select MOD(29,9);
1 rows in set (0.00 sec)

+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+

is displayed like this: 

mysql> select MOD(29,9);
        -> 2

6.3.1 Non-Type-Specific Operators and Functions

6.3.1.1 Parenthesis

( ... )

Use parenthesis to force the order of evaluation in an expression. For example: 

mysql> select 1+2*3;
        -> 7
mysql> select (1+2)*3;
        -> 9

6.3.1.2 Comparison Operators

Comparison operations result in a value of 1 (TRUE), 0 (FALSE), or NULL. These functions work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as needed (as in Perl).

MySQL performs comparisons using the following rules:

By default, string comparisons are done in case-independent fashion using the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English).

The examples below illustrate conversion of strings to numbers for comparison operations: 

mysql> SELECT 1 > '6x';
         -> 0
mysql> SELECT 7 > '6x';
         -> 1
mysql> SELECT 0 > 'x6';
         -> 0
mysql> SELECT 0 = 'x6';
         -> 1
=
Equal: 
mysql> select 1 = 0;
        -> 0
mysql> select '0' = 0;
        -> 1
mysql> select '0.0' = 0;
        -> 1
mysql> select '0.01' = 0;
        -> 0
mysql> select '.01' = 0.01;
        -> 1
<>
!=
Not equal: 
mysql> select '.01' <> '0.01';
        -> 1
mysql> select .01 <> '0.01';
        -> 0
mysql> select 'zapp' <> 'zappp';
        -> 1
<=
Less than or equal: 
mysql> select 0.1 <= 2;
        -> 1
<
Less than: 
mysql> select 2 < 2;
        -> 0
>=
Greater than or equal: 
mysql> select 2 >= 2;
        -> 1
>
Greater than: 
mysql> select 2 > 2;
        -> 0
<=>
Null safe equal: 
mysql> select 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
        -> 1 1 0
IS NULL
IS NOT NULL
Test whether or not a value is or is not NULL: 
mysql> select 1 IS NULL, 0 IS NULL, NULL IS NULL;
        -> 0 0 1
mysql> select 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
        -> 1 1 0
expr BETWEEN min AND max
If expr is greater than or equal to min and expr is less than or equal to max, BETWEEN returns 1, otherwise it returns 0. This is equivalent to the expression (min <= expr AND expr <= max) if all the arguments are of the same type. The first argument (expr) determines how the comparison is performed as follows: 
mysql> select 1 BETWEEN 2 AND 3;
        -> 0
mysql> select 'b' BETWEEN 'a' AND 'c';
        -> 1
mysql> select 2 BETWEEN 2 AND '3';
        -> 1
mysql> select 2 BETWEEN 2 AND 'x-3';
        -> 0
expr IN (value,...)
Returns 1 if expr is any of the values in the IN list, else returns 0. If all values are constants, then all values are evaluated according to the type of expr and sorted. The search for the item is then done using a binary search. This means IN is very quick if the IN value list consists entirely of constants. If expr is a case-sensitive string expression, the string comparison is performed in case-sensitive fashion: 
mysql> select 2 IN (0,3,5,'wefwf');
        -> 0
mysql> select 'wefwf' IN (0,3,5,'wefwf');
        -> 1
expr NOT IN (value,...)
Same as NOT (expr IN (value,...)).
ISNULL(expr)
If expr is NULL, ISNULL() returns 1, otherwise it returns 0: 
mysql> select ISNULL(1+1);
        -> 0
mysql> select ISNULL(1/0);
        -> 1
Note that a comparison of NULL values using = will always be false!
COALESCE(list)
Returns first non-NULL element in list: 
mysql> select COALESCE(NULL,1);
        -> 1
mysql> select COALESCE(NULL,NULL,NULL);
        -> NULL
INTERVAL(N,N1,N2,N3,...)
Returns 0 if N < N1, 1 if N < N2 and so on. All arguments are treated as integers. It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. This is because a binary search is used (very fast): 
mysql> select INTERVAL(23, 1, 15, 17, 30, 44, 200);
        -> 3
mysql> select INTERVAL(10, 1, 10, 100, 1000);
        -> 2
mysql> select INTERVAL(22, 23, 30, 44, 200);
        -> 0

If you are comparing case sensitive string with any of the standard operators (=, <>..., but not LIKE) end space will be ignored. 

mysql> select "a" ="A ";
        -> 1

6.3.1.3 Logical Operators

All logical functions return 1 (TRUE), 0 (FALSE) or NULL (unknown, which is in most cases the same as FALSE):

NOT
!
Logical NOT. Returns 1 if the argument is 0, otherwise returns 0. Exception: NOT NULL returns NULL: 
mysql> select NOT 1;
        -> 0
mysql> select NOT NULL;
        -> NULL
mysql> select ! (1+1);
        -> 0
mysql> select ! 1+1;
        -> 1
The last example returns 1 because the expression evaluates the same way as (!1)+1.
OR
||
Logical OR. Returns 1 if either argument is not 0 and not NULL: 
mysql> select 1 || 0;
        -> 1
mysql> select 0 || 0;
        -> 0
mysql> select 1 || NULL;
        -> 1
AND
&&
Logical AND. Returns 0 if either argument is 0 or NULL, otherwise returns 1: 
mysql> select 1 && NULL;
        -> 0
mysql> select 1 && 0;
        -> 0

6.3.1.4 Control Flow Functions

IFNULL(expr1,expr2)
If expr1 is not NULL, IFNULL() returns expr1, else it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used: 
mysql> select IFNULL(1,0);
        -> 1
mysql> select IFNULL(NULL,10);
        -> 10
mysql> select IFNULL(1/0,10);
        -> 10
mysql> select IFNULL(1/0,'yes');
        -> 'yes'
NULLIF(expr1,expr2)
If expr1 = expr2 is true, return NULL else return expr1. This is the same as CASE WHEN x = y THEN NULL ELSE x END: 
mysql> select NULLIF(1,1);
        -> NULL
mysql> select NULLIF(1,2);
        -> 1
Note that expr1 is evaluated twice in MySQL if the arguments are equal.
IF(expr1,expr2,expr3)
If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then IF() returns expr2, else it returns expr3. IF() returns a numeric or string value, depending on the context in which it is used: 
mysql> select IF(1>2,2,3);
        -> 3
mysql> select IF(1<2,'yes','no');
        -> 'yes'
mysql> select IF(strcmp('test','test1'),'no','yes');
        -> 'no'
expr1 is evaluated as an integer value, which means that if you are testing floating-point or string values, you should do so using a comparison operation: 
mysql> select IF(0.1,1,0);
        -> 0
mysql> select IF(0.1<>0,1,0);
        -> 1
In the first case above, IF(0.1) returns 0 because 0.1 is converted to an integer value, resulting in a test of IF(0). This may not be what you expect. In the second case, the comparison tests the original floating-point value to see whether it is non-zero. The result of the comparison is used as an integer. The default return type of IF() (which may matter when it is stored into a temporary table) is calculated in MySQL Version 3.23 as follows:
Expression Return value
expr2 or expr3 returns string string
expr2 or expr3 returns a floating-point value floating-point
expr2 or expr3 returns an integer integer
CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END
The first version returns the result where value=compare-value. The second version returns the result for the first condition, which is true. If there was no matching result value, then the result after ELSE is returned. If there is no ELSE part then NULL is returned: 
mysql> SELECT CASE 1 WHEN 1 THEN "one" WHEN 2 THEN "two" ELSE "more" END;
       -> "one"
mysql> SELECT CASE WHEN 1>0 THEN "true" ELSE "false" END;
       -> "true"
mysql> SELECT CASE BINARY "B" when "a" then 1 when "b" then 2 END;
       -> NULL

The type of the return value (INTEGER, DOUBLE or STRING) is the same as the type of the first returned value (the expression after the first THEN).

6.3.2 String Functions

String-valued functions return NULL if the length of the result would be greater than the max_allowed_packet server parameter. See section 5.5.2 Tuning Server Parameters.

For functions that operate on string positions, the first position is numbered 1.

ASCII(str)
Returns the ASCII code value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL: 
mysql> select ASCII('2');
        -> 50
mysql> select ASCII(2);
        -> 50
mysql> select ASCII('dx');
        -> 100
See also the ORD() function.
ORD(str)
If the leftmost character of the string str is a multi-byte character, returns the code of multi-byte character by returning the ASCII code value of the character in the format of: ((first byte ASCII code)*256+(second byte ASCII code))[*256+third byte ASCII code...]. If the leftmost character is not a multi-byte character, returns the same value as the like ASCII() function does: 
mysql> select ORD('2');
        -> 50
CONV(N,from_base,to_base)
Converts numbers between different number bases. Returns a string representation of the number N, converted from base from_base to base to_base. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If to_base is a negative number, N is regarded as a signed number. Otherwise, N is treated as unsigned. CONV works with 64-bit precision: 
mysql> select CONV("a",16,2);
        -> '1010'
mysql> select CONV("6E",18,8);
        -> '172'
mysql> select CONV(-17,10,-18);
        -> '-H'
mysql> select CONV(10+"10"+'10'+0xa,10,10);
        -> '40'
BIN(N)
Returns a string representation of the binary value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,2). Returns NULL if N is NULL: 
mysql> select BIN(12);
        -> '1100'
OCT(N)
Returns a string representation of the octal value of N, where N is a longlong number. This is equivalent to CONV(N,10,8). Returns NULL if N is NULL: 
mysql> select OCT(12);
        -> '14'
HEX(N)
Returns a string representation of the hexadecimal value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,16). Returns NULL if N is NULL: 
mysql> select HEX(255);
        -> 'FF'
CHAR(N,...)
CHAR() interprets the arguments as integers and returns a string consisting of the characters given by the ASCII code values of those integers. NULL values are skipped: 
mysql> select CHAR(77,121,83,81,'76');
        -> 'MySQL'
mysql> select CHAR(77,77.3,'77.3');
        -> 'MMM'
CONCAT(str1,str2,...)
Returns the string that results from concatenating the arguments. Returns NULL if any argument is NULL. May have more than 2 arguments. A numeric argument is converted to the equivalent string form: 
mysql> select CONCAT('My', 'S', 'QL');
        -> 'MySQL'
mysql> select CONCAT('My', NULL, 'QL');
        -> NULL
mysql> select CONCAT(14.3);
        -> '14.3'
CONCAT_WS(separator, str1, str2,...)
CONCAT_WS() stands for CONCAT With Separator and is a special form of CONCAT(). The first argument is the separator for the rest of the arguments. The separator can be a string as well as the rest of the arguments. If the separator is NULL, the result will be NULL. The function will skip any NULLs and empty strings, after the separator argument. The separator will be added between the strings to be concatenated: 
mysql> select CONCAT_WS(",","First name","Second name","Last Name");
       -> 'First name,Second name,Last Name'
mysql> select CONCAT_WS(",","First name",NULL,"Last Name");
       -> 'First name,Last Name'
LENGTH(str)
OCTET_LENGTH(str)
CHAR_LENGTH(str)
CHARACTER_LENGTH(str)
Returns the length of the string str: 
mysql> select LENGTH('text');
        -> 4
mysql> select OCTET_LENGTH('text');
        -> 4
Note that for CHAR_LENGTH(), multi-byte characters are only counted once.
LOCATE(substr,str)
POSITION(substr IN str)
Returns the position of the first occurrence of substring substr in string str. Returns 0 if substr is not in str: 
mysql> select LOCATE('bar', 'foobarbar');
        -> 4
mysql> select LOCATE('xbar', 'foobar');
        -> 0
This function is multi-byte safe.
LOCATE(substr,str,pos)
Returns the position of the first occurrence of substring substr in string str, starting at position pos. Returns 0 if substr is not in str: 
mysql> select LOCATE('bar', 'foobarbar',5);
        -> 7
This function is multi-byte safe.
INSTR(str,substr)
Returns the position of the first occurrence of substring substr in string str. This is the same as the two-argument form of LOCATE(), except that the arguments are swapped: 
mysql> select INSTR('foobarbar', 'bar');
        -> 4
mysql> select INSTR('xbar', 'foobar');
        -> 0
This function is multi-byte safe.
LPAD(str,len,padstr)
Returns the string str, left-padded with the string padstr until str is len characters long. If str is longer than len' then it will be shortened to len characters. 
mysql> select LPAD('hi',4,'??');
        -> '??hi'
RPAD(str,len,padstr)
Returns the string str, right-padded with the string padstr until str is len characters long. If str is longer than len' then it will be shortened to len characters. 
mysql> select RPAD('hi',5,'?');
        -> 'hi???'
LEFT(str,len)
Returns the leftmost len characters from the string str: 
mysql> select LEFT('foobarbar', 5);
        -> 'fooba'
This function is multi-byte safe.
RIGHT(str,len)
Returns the rightmost len characters from the string str: 
mysql> select RIGHT('foobarbar', 4);
        -> 'rbar'
This function is multi-byte safe.
SUBSTRING(str,pos,len)
SUBSTRING(str FROM pos FOR len)
MID(str,pos,len)
Returns a substring len characters long from string str, starting at position pos. The variant form that uses FROM is ANSI SQL92 syntax: 
mysql> select SUBSTRING('Quadratically',5,6);
        -> 'ratica'
This function is multi-byte safe.
SUBSTRING(str,pos)
SUBSTRING(str FROM pos)
Returns a substring from string str starting at position pos: 
mysql> select SUBSTRING('Quadratically',5);
        -> 'ratically'
mysql> select SUBSTRING('foobarbar' FROM 4);
        -> 'barbar'
This function is multi-byte safe.
SUBSTRING_INDEX(str,delim,count)
Returns the substring from string str before count occurrences of the delimiter delim. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned: 
mysql> select SUBSTRING_INDEX('www.mysql.com', '.', 2);
        -> 'www.mysql'
mysql> select SUBSTRING_INDEX('www.mysql.com', '.', -2);
        -> 'mysql.com'
This function is multi-byte safe.
LTRIM(str)
Returns the string str with leading space characters removed: 
mysql> select LTRIM('  barbar');
        -> 'barbar'
RTRIM(str)
Returns the string str with trailing space characters removed: 
mysql> select RTRIM('barbar   ');
        -> 'barbar'
This function is multi-byte safe.
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
Returns the string str with all remstr prefixes and/or suffixes removed. If none of the specifiers BOTH, LEADING or TRAILING are given, BOTH is assumed. If remstr is not specified, spaces are removed: 
mysql> select TRIM('  bar   ');
        -> 'bar'
mysql> select TRIM(LEADING 'x' FROM 'xxxbarxxx');
        -> 'barxxx'
mysql> select TRIM(BOTH 'x' FROM 'xxxbarxxx');
        -> 'bar'
mysql> select TRIM(TRAILING 'xyz' FROM 'barxxyz');
        -> 'barx'
This function is multi-byte safe.
SOUNDEX(str)
Returns a soundex string from str. Two strings that sound almost the same should have identical soundex strings. A standard soundex string is 4 characters long, but the SOUNDEX() function returns an arbitrarily long string. You can use SUBSTRING() on the result to get a standard soundex string. All non-alphanumeric characters are ignored in the given string. All international alpha characters outside the A-Z range are treated as vowels: 
mysql> select SOUNDEX('Hello');
        -> 'H400'
mysql> select SOUNDEX('Quadratically');
        -> 'Q36324'
SPACE(N)
Returns a string consisting of N space characters: 
mysql> select SPACE(6);
        -> '      '
REPLACE(str,from_str,to_str)
Returns the string str with all all occurrences of the string from_str replaced by the string to_str: 
mysql> select REPLACE('www.mysql.com', 'w', 'Ww');
        -> 'WwWwWw.mysql.com'
This function is multi-byte safe.
REPEAT(str,count)
Returns a string consisting of the string str repeated count times. If count <= 0, returns an empty string. Returns NULL if str or count are NULL: 
mysql> select REPEAT('MySQL', 3);
        -> 'MySQLMySQLMySQL'
REVERSE(str)
Returns the string str with the order of the characters reversed: 
mysql> select REVERSE('abc');
        -> 'cba'
This function is multi-byte safe.
INSERT(str,pos,len,newstr)
Returns the string str, with the substring beginning at position pos and len characters long replaced by the string newstr: 
mysql> select INSERT('Quadratic', 3, 4, 'What');
        -> 'QuWhattic'
This function is multi-byte safe.
ELT(N,str1,str2,str3,...)
Returns str1 if N = 1, str2 if N = 2, and so on. Returns NULL if N is less than 1 or greater than the number of arguments. ELT() is the complement of FIELD(): 
mysql> select ELT(1, 'ej', 'Heja', 'hej', 'foo');
        -> 'ej'
mysql> select ELT(4, 'ej', 'Heja', 'hej', 'foo');
        -> 'foo'
FIELD(str,str1,str2,str3,...)
Returns the index of str in the str1, str2, str3, ... list. Returns 0 if str is not found. FIELD() is the complement of ELT(): 
mysql> select FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 2
mysql> select FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 0
FIND_IN_SET(str,strlist)
Returns a value 1 to N if the string str is in the list strlist consisting of N substrings. A string list is a string composed of substrings separated by `,' characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic! Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function will not work properly if the first argument contains a `,': 
mysql> SELECT FIND_IN_SET('b','a,b,c,d');
        -> 2
MAKE_SET(bits,str1,str2,...)
Returns a set (a string containing substrings separated by `,' characters) consisting of the strings that have the corresponding bit in bits set. str1 corresponds to bit 0, str2 to bit 1, etc. NULL strings in str1, str2, ... are not appended to the result: 
mysql> SELECT MAKE_SET(1,'a','b','c');
        -> 'a'
mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world');
        -> 'hello,world'
mysql> SELECT MAKE_SET(0,'a','b','c');
        -> ''
EXPORT_SET(bits,on,off,[separator,[number_of_bits]])
Returns a string where for every bit set in 'bit', you get an 'on' string and for every reset bit you get an 'off' string. Each string is separated with 'separator' (default ',') and only 'number_of_bits' (default 64) of 'bits' is used: 
mysql> select EXPORT_SET(5,'Y','N',',',4)
        -> Y,N,Y,N
LCASE(str)
LOWER(str)
Returns the string str with all characters changed to lowercase according to the current character set mapping (the default is ISO-8859-1 Latin1): 
mysql> select LCASE('QUADRATICALLY');
        -> 'quadratically'
This function is multi-byte safe.
UCASE(str)
UPPER(str)
Returns the string str with all characters changed to uppercase according to the current character set mapping (the default is ISO-8859-1 Latin1): 
mysql> select UCASE('Hej');
        -> 'HEJ'
This function is multi-byte safe.
LOAD_FILE(file_name)
Reads the file and returns the file contents as a string. The file must be on the server, you must specify the full pathname to the file, and you must have the file privilege. The file must be readable by all and be smaller than max_allowed_packet. If the file doesn't exist or can't be read due to one of the above reasons, the function returns NULL: 
mysql> UPDATE table_name
           SET blob_column=LOAD_FILE("/tmp/picture")
           WHERE id=1;

If you are not using MySQL Version 3.23, you have to do the reading of the file inside your application and create an INSERT statement to update the database with the file information. One way to do this, if you are using the MySQL++ library, can be found at http://www.mysql.com/documentation/mysql++/mysql++-examples.html.

MySQL automatically converts numbers to strings as necessary, and vice-versa: 

mysql> SELECT 1+"1";
        -> 2
mysql> SELECT CONCAT(2,' test');
        -> '2 test'

If you want to convert a number to a string explicitly, pass it as the argument to CONCAT().

If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This only affects comparisons.

6.3.2.1 String Comparison Functions

Normally, if any expression in a string comparison is case sensitive, the comparison is performed in case-sensitive fashion.

expr LIKE pat [ESCAPE 'escape-char']
Pattern matching using SQL simple regular expression comparison. Returns 1 (TRUE) or 0 (FALSE). With LIKE you can use the following two wild-card characters in the pattern:
% Matches any number of characters, even zero characters
_ Matches exactly one character 
mysql> select 'David!' LIKE 'David_';
        -> 1
mysql> select 'David!' LIKE '%D%v%';
        -> 1
To test for literal instances of a wild-card character, precede the character with the escape character. If you don't specify the ESCAPE character, `\' is assumed:
\% Matches one % character
\_ Matches one _ character 
mysql> select 'David!' LIKE 'David\_';
        -> 0
mysql> select 'David_' LIKE 'David\_';
        -> 1
To specify a different escape character, use the ESCAPE clause: 
mysql> select 'David_' LIKE 'David|_' ESCAPE '|';
        -> 1
The following two statements illustrate that string comparisons are case insensitive unless one of the operands is a binary string: 
mysql> select 'abc' LIKE 'ABC';
        -> 1
mysql> SELECT 'abc' LIKE BINARY 'ABC';
        -> 0
LIKE is allowed on numeric expressions! (This is a MySQL extension to the ANSI SQL LIKE.) 
mysql> select 10 LIKE '1%';
        -> 1
Note: Because MySQL uses the C escape syntax in strings (for example, `\n'), you must double any `\' that you use in your LIKE strings. For example, to search for `\n', specify it as `\\n'. To search for `\', specify it as `\\\\' (the backslashes are stripped once by the parser and another time when the pattern match is done, leaving a single backslash to be matched).
expr NOT LIKE pat [ESCAPE 'escape-char']
Same as NOT (expr LIKE pat [ESCAPE 'escape-char']).
expr REGEXP pat
expr RLIKE pat
Performs a pattern match of a string expression expr against a pattern pat. The pattern can be an extended regular expression. See section I Description of MySQL regular expression syntax. Returns 1 if expr matches pat, otherwise returns 0. RLIKE is a synonym for REGEXP, provided for mSQL compatibility. Note: Because MySQL uses the C escape syntax in strings (for example, `\n'), you must double any `\' that you use in your REGEXP strings. As of MySQL Version 3.23.4, REGEXP is case insensitive for normal (not binary) strings: 
mysql> select 'Monty!' REGEXP 'm%y%%';
        -> 0
mysql> select 'Monty!' REGEXP '.*';
        -> 1
mysql> select 'new*\n*line' REGEXP 'new\\*.\\*line';
        -> 1
mysql> select "a" REGEXP "A", "a" REGEXP BINARY "A";
        -> 1  0
mysql> select "a" REGEXP "^[a-d]";
        -> 1
REGEXP and RLIKE use the current character set (ISO-8859-1 Latin1 by default) when deciding the type of a character.
expr NOT REGEXP pat
expr NOT RLIKE pat
Same as NOT (expr REGEXP pat).
STRCMP(expr1,expr2)
STRCMP() returns 0 if the strings are the same, -1 if the first argument is smaller than the second according to the current sort order, and 1 otherwise: 
mysql> select STRCMP('text', 'text2');
        -> -1
mysql> select STRCMP('text2', 'text');
        -> 1
mysql> select STRCMP('text', 'text');
        -> 0
MATCH (col1,col2,...) AGAINST (expr)
MATCH ... AGAINST() is used for full-text search and returns relevance - similarity measure between the text in columns (col1,col2,...) and the query expr. Relevance is a positive floating-point number. Zero relevance means no similarity. For MATCH ... AGAINST() to work, a FULLTEXT index must be created first. See section 6.5.3 CREATE TABLE Syntax. MATCH ... AGAINST() is available in MySQL Version 3.23.23 or later. For details and usage examples see section 6.9 MySQL Full-text Search.

6.3.2.2 Case Sensitivity

BINARY
The BINARY operator casts the string following it to a binary string. This is an easy way to force a column comparison to be case sensitive even if the column isn't defined as BINARY or BLOB: 
mysql> select "a" = "A";
        -> 1
mysql> select BINARY "a" = "A";
        -> 0
BINARY was introduced in MySQL Version 3.23.0. Note that in some context MySQL will not be able to use the index efficiently when you cast an indexed column to BINARY.

If you want to compare a blob case-insensitively you can always convert the blob to upper case before doing the comparison: 

SELECT 'A' LIKE UPPER(blob_col) FROM table_name;

We plan to soon introduce casting between different character sets to make string comparison even more flexible.

6.3.3 Numeric Functions

6.3.3.1 Arithmetic Operations

The usual arithmetic operators are available. Note that in the case of `-', `+', and `*', the result is calculated with BIGINT (64-bit) precision if both arguments are integers!

+
Addition: 
mysql> select 3+5;
        -> 8
-
Subtraction: 
mysql> select 3-5;
        -> -2
*
Multiplication: 
mysql> select 3*5;
        -> 15
mysql> select 18014398509481984*18014398509481984.0;
        -> 324518553658426726783156020576256.0
mysql> select 18014398509481984*18014398509481984;
        -> 0
The result of the last expression is incorrect because the result of the integer multiplication exceeds the 64-bit range of BIGINT calculations.
/
Division: 
mysql> select 3/5;
        -> 0.60
Division by zero produces a NULL result: 
mysql> select 102/(1-1);
        -> NULL
A division will be calculated with BIGINT arithmetic only if performed in a context where its result is converted to an integer!

6.3.3.2 Mathematical Functions

All mathematical functions return NULL in case of an error.

-
Unary minus. Changes the sign of the argument: 
mysql> select - 2;
        -> -2
Note that if this operator is used with a BIGINT, the return value is a BIGINT! This means that you should avoid using - on integers that may have the value of -2^63!
ABS(X)
Returns the absolute value of X: 
mysql> select ABS(2);
        -> 2
mysql> select ABS(-32);
        -> 32
This function is safe to use with BIGINT values.
SIGN(X)
Returns the sign of the argument as -1, 0, or 1, depending on whether X is negative, zero, or positive: 
mysql> select SIGN(-32);
        -> -1
mysql> select SIGN(0);
        -> 0
mysql> select SIGN(234);
        -> 1
MOD(N,M)
%
Modulo (like the % operator in C). Returns the remainder of N divided by M: 
mysql> select MOD(234, 10);
        -> 4
mysql> select 253 % 7;
        -> 1
mysql> select MOD(29,9);
        -> 2
This function is safe to use with BIGINT values.
FLOOR(X)
Returns the largest integer value not greater than X: 
mysql> select FLOOR(1.23);
        -> 1
mysql> select FLOOR(-1.23);
        -> -2
Note that the return value is converted to a BIGINT!
CEILING(X)
Returns the smallest integer value not less than X: 
mysql> select CEILING(1.23);
        -> 2
mysql> select CEILING(-1.23);
        -> -1
Note that the return value is converted to a BIGINT!
ROUND(X)
Returns the argument X, rounded to the nearest integer: 
mysql> select ROUND(-1.23);
        -> -1
mysql> select ROUND(-1.58);
        -> -2
mysql> select ROUND(1.58);
        -> 2
Note that the behavior of ROUND() when the argument is half way between two integers depends on the C library implementation. Some round to the nearest even number, always up, always down, or always towards zero. If you need one kind of rounding, you should use a well-defined function like TRUNCATE() or FLOOR() instead.
ROUND(X,D)
Returns the argument X, rounded to a number with D decimals. If D is 0, the result will have no decimal point or fractional part: 
mysql> select ROUND(1.298, 1);
        -> 1.3
mysql> select ROUND(1.298, 0);
        -> 1
EXP(X)
Returns the value of e (the base of natural logarithms) raised to the power of X: 
mysql> select EXP(2);
        -> 7.389056
mysql> select EXP(-2);
        -> 0.135335
LOG(X)
Returns the natural logarithm of X: 
mysql> select LOG(2);
        -> 0.693147
mysql> select LOG(-2);
        -> NULL
If you want the log of a number X to some arbitary base B, use the formula LOG(X)/LOG(B).
LOG10(X)
Returns the base-10 logarithm of X: 
mysql> select LOG10(2);
        -> 0.301030
mysql> select LOG10(100);
        -> 2.000000
mysql> select LOG10(-100);
        -> NULL
POW(X,Y)
POWER(X,Y)
Returns the value of X raised to the power of Y: 
mysql> select POW(2,2);
        -> 4.000000
mysql> select POW(2,-2);
        -> 0.250000
SQRT(X)
Returns the non-negative square root of X: 
mysql> select SQRT(4);
        -> 2.000000
mysql> select SQRT(20);
        -> 4.472136
PI()
Returns the value of PI. The default shown number of decimals is 5, but MySQL internally uses the full double precession for PI. 
mysql> select PI();
        -> 3.141593
mysql> SELECT PI()+0.000000000000000000;
        -> 3.141592653589793116
COS(X)
Returns the cosine of X, where X is given in radians: 
mysql> select COS(PI());
        -> -1.000000
SIN(X)
Returns the sine of X, where X is given in radians: 
mysql> select SIN(PI());
        -> 0.000000
TAN(X)
Returns the tangent of X, where X is given in radians: 
mysql> select TAN(PI()+1);
        -> 1.557408
ACOS(X)
Returns the arc cosine of X, that is, the value whose cosine is X. Returns NULL if X is not in the range -1 to 1: 
mysql> select ACOS(1);
        -> 0.000000
mysql> select ACOS(1.0001);
        -> NULL
mysql> select ACOS(0);
        -> 1.570796
ASIN(X)
Returns the arc sine of X, that is, the value whose sine is X. Returns NULL if X is not in the range -1 to 1: 
mysql> select ASIN(0.2);
        -> 0.201358
mysql> select ASIN('foo');
        -> 0.000000
ATAN(X)
Returns the arc tangent of X, that is, the value whose tangent is X: 
mysql> select ATAN(2);
        -> 1.107149
mysql> select ATAN(-2);
        -> -1.107149
ATAN2(Y,X)
Returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X, except that the signs of both arguments are used to determine the quadrant of the result: 
mysql> select ATAN(-2,2);
        -> -0.785398
mysql> select ATAN(PI(),0);
        -> 1.570796
COT(X)
Returns the cotangent of X: 
mysql> select COT(12);
        -> -1.57267341
mysql> select COT(0);
        -> NULL
RAND()
RAND(N)
Returns a random floating-point value in the range 0 to 1.0. If an integer argument N is specified, it is used as the seed value: 
mysql> select RAND();
        -> 0.5925
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND();
        -> 0.2079
mysql> select RAND();
        -> 0.7888
You can't use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. In MySQL Version 3.23, you can, however, do: SELECT * FROM table_name ORDER BY RAND() This is useful to get a random sample of a set SELECT * FROM table1,table2 WHERE a=b AND c<d ORDER BY RAND() LIMIT 1000. Note that a RAND() in a WHERE clause will be re-evaluated every time the WHERE is executed.
LEAST(X,Y,...)
With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared using the following rules: 
mysql> select LEAST(2,0);
        -> 0
mysql> select LEAST(34.0,3.0,5.0,767.0);
        -> 3.0
mysql> select LEAST("B","A","C");
        -> "A"
In MySQL versions prior to Version 3.22.5, you can use MIN() instead of LEAST.
GREATEST(X,Y,...)
Returns the largest (maximum-valued) argument. The arguments are compared using the same rules as for LEAST: 
mysql> select GREATEST(2,0);
        -> 2
mysql> select GREATEST(34.0,3.0,5.0,767.0);
        -> 767.0
mysql> select GREATEST("B","A","C");
        -> "C"
In MySQL versions prior to Version 3.22.5, you can use MAX() instead of GREATEST.
DEGREES(X)
Returns the argument X, converted from radians to degrees: 
mysql> select DEGREES(PI());
        -> 180.000000
RADIANS(X)
Returns the argument X, converted from degrees to radians: 
mysql> select RADIANS(90);
        -> 1.570796
TRUNCATE(X,D)
Returns the number X, truncated to D decimals. If D is 0, the result will have no decimal point or fractional part: 
mysql> select TRUNCATE(1.223,1);
        -> 1.2
mysql> select TRUNCATE(1.999,1);
        -> 1.9
mysql> select TRUNCATE(1.999,0);
        -> 1
Note that as decimal numbers are normally not stored as exact numbers in computers, but as double values, you may be fooled by the following result: 
mysql> select TRUNCATE(10.28*100,0);
       -> 1027
The above happens because 10.28 is actually stored as something like 10.2799999999999999.

6.3.4 Date and Time Functions

See section 6.2.2 Date and Time Types for a description of the range of values each type has and the valid formats in which date and time values may be specified.

Here is an example that uses date functions. The query below selects all records with a date_col value from within the last 30 days: 

mysql> SELECT something FROM table
           WHERE TO_DAYS(NOW()) - TO_DAYS(date_col) <= 30;
DAYOFWEEK(date)
Returns the weekday index for date (1 = Sunday, 2 = Monday, ... 7 = Saturday). These index values correspond to the ODBC standard: 
mysql> select DAYOFWEEK('1998-02-03');
        -> 3
WEEKDAY(date)
Returns the weekday index for date (0 = Monday, 1 = Tuesday, ... 6 = Sunday): 
mysql> select WEEKDAY('1997-10-04 22:23:00');
        -> 5
mysql> select WEEKDAY('1997-11-05');
        -> 2
DAYOFMONTH(date)
Returns the day of the month for date, in the range 1 to 31: 
mysql> select DAYOFMONTH('1998-02-03');
        -> 3
DAYOFYEAR(date)
Returns the day of the year for date, in the range 1 to 366: 
mysql> select DAYOFYEAR('1998-02-03');
        -> 34
MONTH(date)
Returns the month for date, in the range 1 to 12: 
mysql> select MONTH('1998-02-03');
        -> 2
DAYNAME(date)
Returns the name of the weekday for date: 
mysql> select DAYNAME("1998-02-05");
        -> 'Thursday'
MONTHNAME(date)
Returns the name of the month for date: 
mysql> select MONTHNAME("1998-02-05");
        -> 'February'
QUARTER(date)
Returns the quarter of the year for date, in the range 1 to 4: 
mysql> select QUARTER('98-04-01');
        -> 2
WEEK(date)
WEEK(date,first)
With a single argument, returns the week for date, in the range 0 to 53 (yes, there may be the beginnings of a week 53), for locations where Sunday is the first day of the week. The two-argument form of WEEK() allows you to specify whether the week starts on Sunday or Monday. The week starts on Sunday if the second argument is 0, on Monday if the second argument is 1: 
mysql> select WEEK('1998-02-20');
        -> 7
mysql> select WEEK('1998-02-20',0);
        -> 7
mysql> select WEEK('1998-02-20',1);
        -> 8
mysql> select WEEK('1998-12-31',1);
        -> 53
YEAR(date)
Returns the year for date, in the range 1000 to 9999: 
mysql> select YEAR('98-02-03');
        -> 1998
YEARWEEK(date)
YEARWEEK(date,first)
Returns year and week for a date. The second arguments works exactly like the second argument to WEEK(). Note that the year may be different from the year in the date argument for the first and the last week of the year: 
mysql> select YEARWEEK('1987-01-01');
        -> 198653
HOUR(time)
Returns the hour for time, in the range 0 to 23: 
mysql> select HOUR('10:05:03');
        -> 10
MINUTE(time)
Returns the minute for time, in the range 0 to 59: 
mysql> select MINUTE('98-02-03 10:05:03');
        -> 5
SECOND(time)
Returns the second for time, in the range 0 to 59: 
mysql> select SECOND('10:05:03');
        -> 3
PERIOD_ADD(P,N)
Adds N months to period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM. Note that the period argument P is not a date value: 
mysql> select PERIOD_ADD(9801,2);
        -> 199803
PERIOD_DIFF(P1,P2)
Returns the number of months between periods P1 and P2. P1 and P2 should be in the format YYMM or YYYYMM. Note that the period arguments P1 and P2 are not date values: 
mysql> select PERIOD_DIFF(9802,199703);
        -> 11
DATE_ADD(date,INTERVAL expr type)
DATE_SUB(date,INTERVAL expr type)
ADDDATE(date,INTERVAL expr type)
SUBDATE(date,INTERVAL expr type)
These functions perform date arithmetic. They are new for MySQL Version 3.22. ADDDATE() and SUBDATE() are synonyms for DATE_ADD() and DATE_SUB(). In MySQL Version 3.23, you can use + and - instead of DATE_ADD() and DATE_SUB() if the expression on the right side is a date or datetime column. (See example) date is a DATETIME or DATE value specifying the starting date. expr is an expression specifying the interval value to be added or subtracted from the starting date. expr is a string; it may start with a `-' for negative intervals. type is a keyword indicating how the expression should be interpreted. The related function EXTRACT(type FROM date) returns the 'type' interval from the date. The following table shows how the type and expr arguments are related:
type value Expected expr format
SECOND SECONDS
MINUTE MINUTES
HOUR HOURS
DAY DAYS
MONTH MONTHS
YEAR YEARS
MINUTE_SECOND "MINUTES:SECONDS"
HOUR_MINUTE "HOURS:MINUTES"
DAY_HOUR "DAYS HOURS"
YEAR_MONTH "YEARS-MONTHS"
HOUR_SECOND "HOURS:MINUTES:SECONDS"
DAY_MINUTE "DAYS HOURS:MINUTES"
DAY_SECOND "DAYS HOURS:MINUTES:SECONDS"
MySQL allows any punctuation delimiter in the expr format. Those shown in the table are the suggested delimiters. If the date argument is a DATE value and your calculations involve only YEAR, MONTH, and DAY parts (that is, no time parts), the result is a DATE value. Otherwise the result is a DATETIME value: 
mysql> SELECT "1997-12-31 23:59:59" + INTERVAL 1 SECOND;
        -> 1998-01-01 00:00:00
mysql> SELECT INTERVAL 1 DAY + "1997-12-31";
        -> 1998-01-01
mysql> SELECT "1998-01-01" - INTERVAL 1 SECOND;
       -> 1997-12-31 23:59:59
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL 1 SECOND);
        -> 1998-01-01 00:00:00
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL 1 DAY);
        -> 1998-01-01 23:59:59
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL "1:1" MINUTE_SECOND);
        -> 1998-01-01 00:01:00
mysql> SELECT DATE_SUB("1998-01-01 00:00:00",
                       INTERVAL "1 1:1:1" DAY_SECOND);
        -> 1997-12-30 22:58:59
mysql> SELECT DATE_ADD("1998-01-01 00:00:00",
                       INTERVAL "-1 10" DAY_HOUR);
        -> 1997-12-30 14:00:00
mysql> SELECT DATE_SUB("1998-01-02", INTERVAL 31 DAY);
        -> 1997-12-02
If you specify an interval value that is too short (does not include all the interval parts that would be expected from the type keyword), MySQL assumes you have left out the leftmost parts of the interval value. For example, if you specify a type of DAY_SECOND, the value of expr is expected to have days, hours, minutes, and seconds parts. If you specify a value like "1:10", MySQL assumes that the days and hours parts are missing and the value represents minutes and seconds. In other words, "1:10" DAY_SECOND is interpreted in such a way that it is equivalent to "1:10" MINUTE_SECOND. This is analogous to the way that MySQL interprets TIME values as representing elapsed time rather than as time of day. Note that if you add or subtract a date value against something that contains a time part, the date value will be automatically converted to a datetime value: 
mysql> select date_add("1999-01-01", interval 1 day);
       -> 1999-01-02
mysql> select date_add("1999-01-01", interval 1 hour);
       -> 1999-01-01 01:00:00
If you use really incorrect dates, the result is NULL. If you add MONTH, YEAR_MONTH, or YEAR and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month: 
mysql> select DATE_ADD('1998-01-30', Interval 1 month);
        -> 1998-02-28
Note from the preceding example that the word INTERVAL and the type keyword are not case sensitive.
EXTRACT(type FROM date)
The EXTRACT() function uses the same kinds of interval type specifiers as DATE_ADD() or DATE_SUB(), but extracts parts from the date rather than performing date arithmetic. 
mysql> SELECT EXTRACT(YEAR FROM "1999-07-02");
       -> 1999
mysql> SELECT EXTRACT(YEAR_MONTH FROM "1999-07-02 01:02:03");
       -> 199907
mysql> SELECT EXTRACT(DAY_MINUTE FROM "1999-07-02 01:02:03");
       -> 20102
TO_DAYS(date)
Given a date date, returns a daynumber (the number of days since year 0): 
mysql> select TO_DAYS(950501);
        -> 728779
mysql> select TO_DAYS('1997-10-07');
        -> 729669
TO_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it doesn't take into account the days that were lost when the calendar was changed.
FROM_DAYS(N)
Given a daynumber N, returns a DATE value: 
mysql> select FROM_DAYS(729669);
        -> '1997-10-07'
FROM_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it doesn't take into account the days that were lost when the calendar was changed.
DATE_FORMAT(date,format)
Formats the date value according to the format string. The following specifiers may be used in the format string:
%M Month name (January..December)
%W Weekday name (Sunday..Saturday)
%D Day of the month with English suffix (1st, 2nd, 3rd, etc.)
%Y Year, numeric, 4 digits
%y Year, numeric, 2 digits
%X Year for the week where Sunday is the first day of the week, numeric, 4 digits, used with '%V'
%x Year for the week, where Monday is the first day of the week, numeric, 4 digits, used with '%v'
%a Abbreviated weekday name (Sun..Sat)
%d Day of the month, numeric (00..31)
%e Day of the month, numeric (0..31)
%m Month, numeric (01..12)
%c Month, numeric (1..12)
%b Abbreviated month name (Jan..Dec)
%j Day of year (001..366)
%H Hour (00..23)
%k Hour (0..23)
%h Hour (01..12)
%I Hour (01..12)
%l Hour (1..12)
%i Minutes, numeric (00..59)
%r Time, 12-hour (hh:mm:ss [AP]M)
%T Time, 24-hour (hh:mm:ss)
%S Seconds (00..59)
%s Seconds (00..59)
%p AM or PM
%w Day of the week (0=Sunday..6=Saturday)
%U Week (0..53), where Sunday is the first day of the week
%u Week (0..53), where Monday is the first day of the week
%V Week (1..53), where Sunday is the first day of the week. Used with '%X'
%v Week (1..53), where Monday is the first day of the week. Used with '%x'
%% A literal `%'.
All other characters are just copied to the result without interpretation: 
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y');
        -> 'Saturday October 1997'
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%H:%i:%s');
        -> '22:23:00'
mysql> select DATE_FORMAT('1997-10-04 22:23:00',
                          '%D %y %a %d %m %b %j');
        -> '4th 97 Sat 04 10 Oct 277'
mysql> select DATE_FORMAT('1997-10-04 22:23:00',
                          '%H %k %I %r %T %S %w');
        -> '22 22 10 10:23:00 PM 22:23:00 00 6'
mysql> select DATE_FORMAT('1999-01-01', '%X %V');
        -> '1998 52'
As of MySQL Version 3.23, the `%' character is required before format specifier characters. In earlier versions of MySQL, `%' was optional.
TIME_FORMAT(time,format)
This is used like the DATE_FORMAT() function above, but the format string may contain only those format specifiers that handle hours, minutes, and seconds. Other specifiers produce a NULL value or 0.
CURDATE()
CURRENT_DATE
Returns today's date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context: 
mysql> select CURDATE();
        -> '1997-12-15'
mysql> select CURDATE() + 0;
        -> 19971215
CURTIME()
CURRENT_TIME
Returns the current time as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select CURTIME();
        -> '23:50:26'
mysql> select CURTIME() + 0;
        -> 235026
NOW()
SYSDATE()
CURRENT_TIMESTAMP
Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select NOW();
        -> '1997-12-15 23:50:26'
mysql> select NOW() + 0;
        -> 19971215235026
UNIX_TIMESTAMP()
UNIX_TIMESTAMP(date)
If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' GMT). If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' GMT. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD in local time: 
mysql> select UNIX_TIMESTAMP();
        -> 882226357
mysql> select UNIX_TIMESTAMP('1997-10-04 22:23:00');
        -> 875996580
When UNIX_TIMESTAMP is used on a TIMESTAMP column, the function will receive the value directly, with no implicit ``string-to-unix-timestamp'' conversion. If you give UNIX_TIMESTAMP() a wrong or out-of-range date, it will return 0.
FROM_UNIXTIME(unix_timestamp)
Returns a representation of the unix_timestamp argument as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select FROM_UNIXTIME(875996580);
        -> '1997-10-04 22:23:00'
mysql> select FROM_UNIXTIME(875996580) + 0;
        -> 19971004222300
FROM_UNIXTIME(unix_timestamp,format)
Returns a string representation of the Unix timestamp, formatted according to the format string. format may contain the same specifiers as those listed in the entry for the DATE_FORMAT() function: 
mysql> select FROM_UNIXTIME(UNIX_TIMESTAMP(),
                            '%Y %D %M %h:%i:%s %x');
        -> '1997 23rd December 03:43:30 x'
SEC_TO_TIME(seconds)
Returns the seconds argument, converted to hours, minutes, and seconds, as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select SEC_TO_TIME(2378);
        -> '00:39:38'
mysql> select SEC_TO_TIME(2378) + 0;
        -> 3938
TIME_TO_SEC(time)
Returns the time argument, converted to seconds: 
mysql> select TIME_TO_SEC('22:23:00');
        -> 80580
mysql> select TIME_TO_SEC('00:39:38');
        -> 2378

6.3.5 Other Functions

6.3.5.1 Bit Functions

MySQL uses BIGINT (64-bit) arithmetic for bit operations, so these operators have a maximum range of 64 bits.

|
Bitwise OR: 
mysql> select 29 | 15;
        -> 31
&
Bitwise AND: 
mysql> select 29 & 15;
        -> 13
<<
Shifts a longlong (BIGINT) number to the left: 
mysql> select 1 << 2;
        -> 4
>>
Shifts a longlong (BIGINT) number to the right: 
mysql> select 4 >> 2;
        -> 1
~
Invert all bits: 
mysql> select 5 & ~1;
        -> 4
BIT_COUNT(N)
Returns the number of bits that are set in the argument N: 
mysql> select BIT_COUNT(29);
        -> 4

6.3.5.2 Miscellaneous Functions

DATABASE()
Returns the current database name: 
mysql> select DATABASE();
        -> 'test'
If there is no current database, DATABASE() returns the empty string.
USER()
SYSTEM_USER()
SESSION_USER()
Returns the current MySQL user name: 
mysql> select USER();
        -> 'davida@localhost'
In MySQL Version 3.22.11 or later, this includes the client hostname as well as the user name. You can extract just the user name part like this (which works whether or not the value includes a hostname part): 
mysql> select substring_index(USER(),"@",1);
        -> 'davida'
PASSWORD(str)
Calculates a password string from the plaintext password str. This is the function that is used for encrypting MySQL passwords for storage in the Password column of the user grant table: 
mysql> select PASSWORD('badpwd');
        -> '7f84554057dd964b'
PASSWORD() encryption is non-reversible. PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, PASSWORD() will result in the same encrypted value as is stored in the Unix password file. See ENCRYPT().
ENCRYPT(str[,salt])
Encrypt str using the Unix crypt() system call. The salt argument should be a string with two characters. (As of MySQL Version 3.22.16, salt may be longer than two characters.): 
mysql> select ENCRYPT("hello");
        -> 'VxuFAJXVARROc'
If crypt() is not available on your system, ENCRYPT() always returns NULL. ENCRYPT() ignores all but the first 8 characters of str, at least on some systems. This will be determined by the behavior of the underlying crypt() system call.
ENCODE(str,pass_str)
Encrypt str using pass_str as the password. To decrypt the result, use DECODE(). The results is a binary string of the same length as string. If you want to save it in a column, use a BLOB column type.
DECODE(crypt_str,pass_str)
Descrypts the encrypted string crypt_str using pass_str as the password. crypt_str should be a string returned from ENCODE().
MD5(string)
Calculates a MD5 checksum for the string. Value is returned as a 32 long hex number that may, for example, be used as a hash key: 
mysql> select MD5("testing");
        -> 'ae2b1fca515949e5d54fb22b8ed95575'
This is an "RSA Data Security, Inc. MD5 Message-Digest Algorithm".
LAST_INSERT_ID([expr])
Returns the last automatically generated value that was inserted into an AUTO_INCREMENT column. See section 8.4.3.126 mysql_insert_id(). 
mysql> select LAST_INSERT_ID();
        -> 195
The last ID that was generated is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another AUTO_INCREMENT column with a non-magic value (that is, a value that is not NULL and not 0). If you insert many rows at the same time with an insert statement, LAST_INSERT_ID() returns the value for the first inserted row. The reason for this is so that you it makes it possible to easily reproduce the same INSERT statement against some other server. If expr is given as an argument to LAST_INSERT_ID(), then the value of the argument is returned by the function, is set as the next value to be returned by LAST_INSERT_ID() and used as the next auto_increment value. This can be used to simulate sequences: First create the table: 
mysql> create table sequence (id int not null);
mysql> insert into sequence values (0);
Then the table can be used to generate sequence numbers like this: 
mysql> update sequence set id=LAST_INSERT_ID(id+1);
You can generate sequences without calling LAST_INSERT_ID(), but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value. You can retrieve the new ID as you would read any normal AUTO_INCREMENT value in MySQL. For example, LAST_INSERT_ID() (without an argument) will return the new ID. The C API function mysql_insert_id() can also be used to get the value. Note that as mysql_insert_id() is only updated after INSERT and UPDATE statements, you can't use this function to retrieve the value for LAST_INSERT_ID(expr) after executing other SQL statements like SELECT or SET.
FORMAT(X,D)
Formats the number X to a format like '#,###,###.##', rounded to D decimals. If D is 0, the result will have no decimal point or fractional part: 
mysql> select FORMAT(12332.123456, 4);
        -> '12,332.1235'
mysql> select FORMAT(12332.1,4);
        -> '12,332.1000'
mysql> select FORMAT(12332.2,0);
        -> '12,332'
VERSION()
Returns a string indicating the MySQL server version: 
mysql> select VERSION();
        -> '3.23.13-log'
Note that if your version ends with -log this means that logging is enabled.
CONNECTION_ID()
Returns the connection id (thread_id) for the connection. Every connection has its own unique id: 
mysql> select CONNECTION_ID();
        -> 1
GET_LOCK(str,timeout)
Tries to obtain a lock with a name given by the string str, with a timeout of timeout seconds. Returns 1 if the lock was obtained successfully, 0 if the attempt timed out, or NULL if an error occurred (such as running out of memory or the thread was killed with mysqladmin kill). A lock is released when you execute RELEASE_LOCK(), execute a new GET_LOCK(), or the thread terminates. This function can be used to implement application locks or to simulate record locks. It blocks requests by other clients for locks with the same name; clients that agree on a given lock string name can use the string to perform cooperative advisory locking: 
mysql> select GET_LOCK("lock1",10);
        -> 1
mysql> select GET_LOCK("lock2",10);
        -> 1
mysql> select RELEASE_LOCK("lock2");
        -> 1
mysql> select RELEASE_LOCK("lock1");
        -> NULL
Note that the second RELEASE_LOCK() call returns NULL because the lock "lock1" was automatically released by the second GET_LOCK() call.
RELEASE_LOCK(str)
Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock wasn't locked by this thread (in which case the lock is not released), and NULL if the named lock didn't exist. The lock will not exist if it was never obtained by a call to GET_LOCK() or if it already has been released.
BENCHMARK(count,expr)
The BENCHMARK() function executes the expression expr repeatedly count times. It may be used to time how fast MySQL processes the expression. The result value is always 0. The intended use is in the mysql client, which reports query execution times: 
mysql> select BENCHMARK(1000000,encode("hello","goodbye"));
+----------------------------------------------+
| BENCHMARK(1000000,encode("hello","goodbye")) |
+----------------------------------------------+
|                                            0 |
+----------------------------------------------+
1 row in set (4.74 sec)
The time reported is elapsed time on the client end, not CPU time on the server end. It may be advisable to execute BENCHMARK() several times, and interpret the result with regard to how heavily loaded the server machine is.
INET_NTOA(expr)
Returns the network address (4 or 8 byte) for the numeric expression: 
mysql> select INET_NTOA(3520061480);
       ->  "209.207.224.40"
INET_ATON(expr)
Returns an integer that represents the numeric value for a network address. Addresses may be 4 or 8 byte addresses: 
mysql> select INET_ATON("209.207.224.40");
       ->  3520061480
The generated number is always in network byte order; For example the above number is calculated as 209*255^3 + 207*255^2 + 224*255 +40.
MASTER_POS_WAIT(log_name, log_pos)
Blocks until the slave reaches the specified position in the master log during replication. If master information is not initialized, returns NULL. If the slave is not running, will block and wait until it is started and goes to or past the specified position. If the slave is already past the specified position, returns immediately. The return value is the number of log events it had to wait to get to the specified position, or NULL in case of error. Useful for control of master-slave synchronization, but was originally written to facilitate replication testing.

6.4 Data Manipulation: SELECT, INSERT, UPDATE, DELETE

6.4.1 SELECT Syntax

SELECT [STRAIGHT_JOIN] [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
       [HIGH_PRIORITY]
       [DISTINCT | DISTINCTROW | ALL]
    select_expression,...
    [INTO {OUTFILE | DUMPFILE} 'file_name' export_options]
    [FROM table_references
        [WHERE where_definition]
        [GROUP BY {unsigned_integer | col_name | formula} [ASC | DESC], ...]
        [HAVING where_definition]
        [ORDER BY {unsigned_integer | col_name | formula} [ASC | DESC] ,...]
        [LIMIT [offset,] rows]
        [PROCEDURE procedure_name]
        [FOR UPDATE | LOCK IN SHARE MODE]]

SELECT is used to retrieve rows selected from one or more tables. select_expression indicates the columns you want to retrieve. SELECT may also be used to retrieve rows computed without reference to any table. For example: 

mysql> SELECT 1 + 1;
         -> 2

All keywords used must be given in exactly the order shown above. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.

6.4.1.1 JOIN Syntax

MySQL supports the following JOIN syntaxes for use in SELECT statements: 

table_reference, table_reference
table_reference [CROSS] JOIN table_reference
table_reference INNER JOIN table_reference join_condition
table_reference STRAIGHT_JOIN table_reference
table_reference LEFT [OUTER] JOIN table_reference join_condition
table_reference LEFT [OUTER] JOIN table_reference
table_reference NATURAL [LEFT [OUTER]] JOIN table_reference
{ oj table_reference LEFT OUTER JOIN table_reference ON conditional_expr }
table_reference RIGHT [OUTER] JOIN table_reference join_condition
table_reference RIGHT [OUTER] JOIN table_reference
table_reference NATURAL [RIGHT [OUTER]] JOIN table_reference

Where table_reference is defined as: 

table_name [[AS] alias] [USE INDEX (key_list)] [IGNORE INDEX (key_list)]

and join_condition is defined as: 

ON conditional_expr |
USING (column_list)

You should never have any conditions in the ON part that are used to restrict which rows you have in the result set. If you want to restrict which rows should be in the result, you have to do this in the WHERE clause.

Note that in versions before Version 3.23.17, the INNER JOIN didn't take a join_condition!

The last LEFT OUTER JOIN syntax shown above exists only for compatibility with ODBC:

Some examples: 

mysql> select * from table1,table2 where table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 USING (id);
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id
           LEFT JOIN table3 ON table2.id=table3.id;
mysql> select * from table1 USE INDEX (key1,key2) WHERE key1=1 and key2=2 AND
       key3=3;
mysql> select * from table1 IGNORE INDEX (key3) WHERE key1=1 and key2=2 AND
       key3=3;

See section 5.2.6 How MySQL Optimizes LEFT JOIN and RIGHT JOIN.

6.4.2 UNION Syntax

SELECT ....
UNION [ALL]
SELECT ....
  [UNION 
   SELECT ...]

UNION is implemented in MySQL 4.0.0.

UNION is used to combine the result from many SELECT statements into one result set.

The SELECT commands are normal select commands, but with the following restrictions:

If you don't use the keyword ALL for the UNION, all returned rows will be unique, like if you had done a DISTINCT for the total result set. If you specify ALL, then you will get all matching rows from all the used SELECT statements.

6.4.3 INSERT Syntax

    INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...),(...),...
or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name
        SET col_name=expression, col_name=expression, ...

INSERT inserts new rows into an existing table. The INSERT ... VALUES form of the statement inserts rows based on explicitly specified values. The INSERT ... SELECT form inserts rows selected from another table or tables. The INSERT ... VALUES form with multiple value lists is supported in MySQL Version 3.22.5 or later. The col_name=expression syntax is supported in MySQL Version 3.22.10 or later.

tbl_name is the table into which rows should be inserted. The column name list or the SET clause indicates which columns the statement specifies values for:

If you use INSERT ... SELECT or an INSERT ... VALUES statement with multiple value lists, you can use the C API function mysql_info() to get information about the query. The format of the information string is shown below: 

Records: 100 Duplicates: 0 Warnings: 0

Duplicates indicates the number of rows that couldn't be inserted because they would duplicate some existing unique index value. Warnings indicates the number of attempts to insert column values that were problematic in some way. Warnings can occur under any of the following conditions:

6.4.3.1 INSERT ... SELECT Syntax

INSERT [LOW_PRIORITY] [IGNORE] [INTO] tbl_name [(column list)] SELECT ...

With INSERT ... SELECT statement you can quickly insert many rows into a table from one or many tables. 

INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE
tblTemp1.fldOrder_ID > 100;

The following conditions hold for an INSERT ... SELECT statement:

You can of course also use REPLACE instead of INSERT to overwrite old rows.

6.4.4 INSERT DELAYED syntax

INSERT DELAYED ...

The DELAYED option for the INSERT statement is a MySQL-specific option that is very useful if you have clients that can't wait for the INSERT to complete. This is a common problem when you use MySQL for logging and you also periodically run SELECT and UPDATE statements that take a long time to complete. DELAYED was introduced in MySQL Version 3.22.15. It is a MySQL extension to ANSI SQL92.

INSERT DELAYED only works with ISAM and MyISAM tables. Note that as MyISAM tables supports concurrent SELECT and INSERT, if there is no free blocks in the middle of the data file, you very seldom need to use INSERT DELAYED with MyISAM. See section 7.1 MyISAM Tables.

When you use INSERT DELAYED, the client will get an OK at once and the row will be inserted when the table is not in use by any other thread.

Another major benefit of using INSERT DELAYED is that inserts from many clients are bundled together and written in one block. This is much faster than doing many separate inserts.

Note that currently the queued rows are only stored in memory until they are inserted into the table. This means that if you kill mysqld the hard way (kill -9) or if mysqld dies unexpectedly, any queued rows that weren't written to disk are lost!

The following describes in detail what happens when you use the DELAYED option to INSERT or REPLACE. In this description, the ``thread'' is the thread that received an INSERT DELAYED command and ``handler'' is the thread that handles all INSERT DELAYED statements for a particular table.

Note that INSERT DELAYED is slower than a normal INSERT if the table is not in use. There is also the additional overhead for the server to handle a separate thread for each table on which you use INSERT DELAYED. This means that you should only use INSERT DELAYED when you are really sure you need it!

6.4.5 UPDATE Syntax

UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
    SET col_name1=expr1, [col_name2=expr2, ...]
    [WHERE where_definition]
    [LIMIT #]

UPDATE updates columns in existing table rows with new values. The SET clause indicates which columns to modify and the values they should be given. The WHERE clause, if given, specifies which rows should be updated. Otherwise all rows are updated. If the ORDER BY clause is specified, the rows will be updated in the order that is specified.

If you specify the keyword LOW_PRIORITY, execution of the UPDATE is delayed until no other clients are reading from the table.

If you specify the keyword IGNORE, the update statement will not abort even if we get duplicate key errors during the update. Rows that would cause conflicts will not be updated.

If you access a column from tbl_name in an expression, UPDATE uses the current value of the column. For example, the following statement sets the age column to one more than its current value: 

mysql> UPDATE persondata SET age=age+1;

UPDATE assignments are evaluated from left to right. For example, the following statement doubles the age column, then increments it: 

mysql> UPDATE persondata SET age=age*2, age=age+1;

If you set a column to the value it currently has, MySQL notices this and doesn't update it.

UPDATE returns the number of rows that were actually changed. In MySQL Version 3.22 or later, the C API function mysql_info() returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.

In MySQL Version 3.23, you can use LIMIT # to ensure that only a given number of rows are changed.

6.4.6 DELETE Syntax

DELETE [LOW_PRIORITY | QUICK] FROM table_name
    [WHERE where_definition]
    [ORDER BY ...]
    [LIMIT rows]

or

DELETE [LOW_PRIORITY | QUICK] table_name[.*] [table_name[.*] ...] FROM
table-references [WHERE where_definition]

DELETE deletes rows from table_name that satisfy the condition given by where_definition, and returns the number of records deleted.

If you issue a DELETE with no WHERE clause, all rows are deleted. If you do this in AUTOCOMMIT mode, this works as TRUNCATE. See section 6.4.7 TRUNCATE Syntax. In MySQL 3.23 DELETE without a WHERE clause will return zero as the number of affected records.

If you really want to know how many records are deleted when you are deleting all rows, and are willing to suffer a speed penalty, you can use a DELETE statement of this form: 

mysql> DELETE FROM table_name WHERE 1>0;

Note that this is MUCH slower than DELETE FROM table_name with no WHERE clause, because it deletes rows one at a time.

If you specify the keyword LOW_PRIORITY, execution of the DELETE is delayed until no other clients are reading from the table.

If you specify the word QUICK then the table handler will not merge index leafs during delete, which may speed up certain kind of deletes.

In MyISAM tables deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. To reclaim unused space and reduce file sizes, use the OPTIMIZE TABLE statement or the myisamchk utility to reorganize tables. OPTIMIZE TABLE is easier, but myisamchk is faster. See section 4.5.1 OPTIMIZE TABLE Syntax and section 4.4.6.10 Table Optimization.

The multi table delete format is supported starting from MySQL 4.0.0.

The idea is that only matching rows from the tables listed BEFORE the FROM clause is deleted. The effect is that you can delete rows from many tables at the same time and also have additional tables that are used for searching.

The .* after the table names is there just to be compatible with Access: 

DELETE t1,t2 FROM t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id

In the above case we delete matching rows just from tables t1 and t2.

ORDER BY and using multiple tables in the DELETE is supported in MySQL 4.0.

If an ORDER BY clause is used, the rows will be deleted in that order. This is really only useful in conjunction with LIMIT. For example: 

DELETE FROM somelog
WHERE user = 'jcole'
ORDER BY timestamp
LIMIT 1

This will delete the oldest entry (by timestamp) where the row matches the WHERE clause.

The MySQL-specific LIMIT rows option to DELETE tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a specific DELETE command doesn't take too much time. You can simply repeat the DELETE command until the number of affected rows is less than the LIMIT value.

6.4.7 TRUNCATE Syntax

TRUNCATE TABLE table_name

In 3.23 TRUNCATE TABLE is mapped to COMMIT ; DELETE FROM table_name. See section 6.4.6 DELETE Syntax.

The differences between TRUNCATE TABLE and DELETE FROM .. are:

TRUNCATE is an Oracle SQL extension.

6.4.8 REPLACE Syntax

    REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...),(...),...
or  REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name
        SET col_name=expression, col_name=expression,...

REPLACE works exactly like INSERT, except that if an old record in the table has the same value as a new record on a unique index, the old record is deleted before the new record is inserted. See section 6.4.3 INSERT Syntax.

In other words, you can't access the values of the old row from a REPLACE statement. In some old MySQL version it looked like you could do this, but that was a bug that has been corrected.

When one uses a REPLACE command, mysql_affected_rows() will return 2 if the new row replaced and old row. This is because in this case one row was inserted and then the duplicate was deleted.

The above makes it easy to check if REPLACE added or replaced a row.

6.4.9 LOAD DATA INFILE Syntax

LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name.txt'
    [REPLACE | IGNORE]
    INTO TABLE tbl_name
    [FIELDS
        [TERMINATED BY '\t']
        [[OPTIONALLY] ENCLOSED BY '']
        [ESCAPED BY '\\' ]
    ]
    [LINES TERMINATED BY '\n']
    [IGNORE number LINES]
    [(col_name,...)]

The LOAD DATA INFILE statement reads rows from a text file into a table at a very high speed. If the LOCAL keyword is specified, the file is read from the client host. If LOCAL is not specified, the file must be located on the server. (LOCAL is available in MySQL Version 3.22.6 or later.)

For security reasons, when reading text files located on the server, the files must either reside in the database directory or be readable by all. Also, to use LOAD DATA INFILE on server files, you must have the file privilege on the server host. See section 4.2.6 Privileges Provided by MySQL.

If you specify the keyword LOW_PRIORITY, execution of the LOAD DATA statement is delayed until no other clients are reading from the table.

If you specify the keyword CONCURRENT with a MyISAM table, then other threads can retrieve data from the table while LOAD DATA is executing. Using this option will of course affect the performance of LOAD DATA a bit even if no other thread is using the table at the same time.

Using LOCAL will be a bit slower than letting the server access the files directly, because the contents of the file must travel from the client host to the server host. On the other hand, you do not need the file privilege to load local files.

If you are using MySQL before Version 3.23.24 you can't read from a FIFO with LOAD DATA INFILE. If you need to read from a FIFO (for example the output from gunzip), use LOAD DATA LOCAL INFILE instead.

You can also load data files by using the mysqlimport utility; it operates by sending a LOAD DATA INFILE command to the server. The --local option causes mysqlimport to read data files from the client host. You can specify the --compress option to get better performance over slow networks if the client and server support the compressed protocol.

When locating files on the server host, the server uses the following rules:

Note that these rules mean a file given as `./myfile.txt' is read from the server's data directory, whereas a file given as `myfile.txt' is read from the database directory of the current database. For example, the following LOAD DATA statement reads the file `data.txt' from the database directory for db1 because db1 is the current database, even though the statement explicitly loads the file into a table in the db2 database: 

mysql> USE db1;
mysql> LOAD DATA INFILE "data.txt" INTO TABLE db2.my_table;

The REPLACE and IGNORE keywords control handling of input records that duplicate existing records on unique key values. If you specify REPLACE, new rows replace existing rows that have the same unique key value. If you specify IGNORE, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.

If you load data from a local file using the LOCAL keyword, the server has no way to stop transmission of the file in the middle of the operation, so the default bahavior is the same as if IGNORE is specified.

If you use LOAD DATA INFILE on an empty MyISAM table, all non-unique indexes are created in a separate batch (like in REPAIR). This normally makes LOAD DATA INFILE much faster when you have many indexes.

LOAD DATA INFILE is the complement of SELECT ... INTO OUTFILE. See section 6.4.1 SELECT Syntax. To write data from a database to a file, use SELECT ... INTO OUTFILE. To read the file back into the database, use LOAD DATA INFILE. The syntax of the FIELDS and LINES clauses is the same for both commands. Both clauses are optional, but FIELDS must precede LINES if both are specified.

If you specify a FIELDS clause, each of its subclauses (TERMINATED BY, [OPTIONALLY] ENCLOSED BY, and ESCAPED BY) is also optional, except that you must specify at least one of them.

If you don't specify a FIELDS clause, the defaults are the same as if you had written this: 

FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\'

If you don't specify a LINES clause, the default is the same as if you had written this: 

LINES TERMINATED BY '\n'

In other words, the defaults cause LOAD DATA INFILE to act as follows when reading input:

Conversely, the defaults cause SELECT ... INTO OUTFILE to act as follows when writing output:

Note that to write FIELDS ESCAPED BY '\\', you must specify two backslashes for the value to be read as a single backslash.

The IGNORE number LINES option can be used to ignore a header of column names at the start of the file: 

mysql> LOAD DATA INFILE "/tmp/file_name" into table test IGNORE 1 LINES;

When you use SELECT ... INTO OUTFILE in tandem with LOAD DATA INFILE to write data from a database into a file and then read the file back into the database later, the field and line handling options for both commands must match. Otherwise, LOAD DATA INFILE will not interpret the contents of the file properly. Suppose you use SELECT ... INTO OUTFILE to write a file with fields delimited by commas: 

mysql> SELECT * INTO OUTFILE 'data.txt'
           FIELDS TERMINATED BY ','
           FROM ...;

To read the comma-delimited file back in, the correct statement would be: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY ',';

If instead you tried to read in the file with the statement shown below, it wouldn't work because it instructs LOAD DATA INFILE to look for tabs between fields: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY '\t';

The likely result is that each input line would be interpreted as a single field.

LOAD DATA INFILE can be used to read files obtained from external sources, too. For example, a file in dBASE format will have fields separated by commas and enclosed in double quotes. If lines in the file are terminated by newlines, the command shown below illustrates the field and line handling options you would use to load the file: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name
           FIELDS TERMINATED BY ',' ENCLOSED BY '"'
           LINES TERMINATED BY '\n';

Any of the field or line handling options may specify an empty string (''). If not empty, the FIELDS [OPTIONALLY] ENCLOSED BY and FIELDS ESCAPED BY values must be a single character. The FIELDS TERMINATED BY and LINES TERMINATED BY values may be more than one character. For example, to write lines that are terminated by carriage return-linefeed pairs, or to read a file containing such lines, specify a LINES TERMINATED BY '\r\n' clause.

For example, to read a file of jokes, that are separated with a line of %%, into a SQL table you can do: 

create table jokes (a int not null auto_increment primary key, joke text
not null);
load data infile "/tmp/jokes.txt" into table jokes fields terminated by ""
lines terminated by "\n%%\n" (joke);

FIELDS [OPTIONALLY] ENCLOSED BY controls quoting of fields. For output (SELECT ... INTO OUTFILE), if you omit the word OPTIONALLY, all fields are enclosed by the ENCLOSED BY character. An example of such output (using a comma as the field delimiter) is shown below: 

"1","a string","100.20"
"2","a string containing a , comma","102.20"
"3","a string containing a \" quote","102.20"
"4","a string containing a \", quote and comma","102.20"

If you specify OPTIONALLY, the ENCLOSED BY character is used only to enclose CHAR and VARCHAR fields: 

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a \" quote",102.20
4,"a string containing a \", quote and comma",102.20

Note that occurrences of the ENCLOSED BY character within a field value are escaped by prefixing them with the ESCAPED BY character. Also note that if you specify an empty ESCAPED BY value, it is possible to generate output that cannot be read properly by LOAD DATA INFILE. For example, the output just shown above would appear as shown below if the escape character is empty. Observe that the second field in the fourth line contains a comma following the quote, which (erroneously) appears to terminate the field: 

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a " quote",102.20
4,"a string containing a ", quote and comma",102.20

For input, the ENCLOSED BY character, if present, is stripped from the ends of field values. (This is true whether or not OPTIONALLY is specified; OPTIONALLY has no effect on input interpretation.) Occurrences of the ENCLOSED BY character preceded by the ESCAPED BY character are interpreted as part of the current field value. In addition, duplicated ENCLOSED BY characters occurring within fields are interpreted as single ENCLOSED BY characters if the field itself starts with that character. For example, if ENCLOSED BY '"' is specified, quotes are handled as shown below: 

"The ""BIG"" boss"  -> The "BIG" boss
The "BIG" boss      -> The "BIG" boss
The ""BIG"" boss    -> The ""BIG"" boss

FIELDS ESCAPED BY controls how to write or read special characters. If the FIELDS ESCAPED BY character is not empty, it is used to prefix the following characters on output:

If the FIELDS ESCAPED BY character is empty, no characters are escaped. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.

For input, if the FIELDS ESCAPED BY character is not empty, occurrences of that character are stripped and the following character is taken literally as part of a field value. The exceptions are an escaped `0' or `N' (for example, \0 or \N if the escape character is `\'). These sequences are interpreted as ASCII 0 (a zero-valued byte) and NULL. See below for the rules on NULL handling.

For more information about `\'-escape syntax, see section 6.1.1 Literals: How to Write Strings and Numbers.

In certain cases, field and line handling options interact:

Handling of NULL values varies, depending on the FIELDS and LINES options you use:

Some cases are not supported by LOAD DATA INFILE:

The following example loads all columns of the persondata table: 

mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;

No field list is specified, so LOAD DATA INFILE expects input rows to contain a field for each table column. The default FIELDS and LINES values are used.

If you wish to load only some of a table's columns, specify a field list: 

mysql> LOAD DATA INFILE 'persondata.txt'
           INTO TABLE persondata (col1,col2,...);

You must also specify a field list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match up input fields with table columns.

If a row has too few fields, the columns for which no input field is present are set to default values. Default value assignment is described in section 6.5.3 CREATE TABLE Syntax.

An empty field value is interpreted differently than if the field value is missing:

Note that these are the same values that result if you assign an empty string explicitly to a string, numeric, or date or time type explicitly in an INSERT or UPDATE statement.

TIMESTAMP columns are only set to the current date and time if there is a NULL value for the column, or (for the first TIMESTAMP column only) if the TIMESTAMP column is left out from the field list when a field list is specified.

If an input row has too many fields, the extra fields are ignored and the number of warnings is incremented.

LOAD DATA INFILE regards all input as strings, so you can't use numeric values for ENUM or SET columns the way you can with INSERT statements. All ENUM and SET values must be specified as strings!

If you are using the C API, you can get information about the query by calling the API function mysql_info() when the LOAD DATA INFILE query finishes. The format of the information string is shown below: 

Records: 1  Deleted: 0  Skipped: 0  Warnings: 0

Warnings occur under the same circumstances as when values are inserted via the INSERT statement (see section 6.4.3 INSERT Syntax), except that LOAD DATA INFILE also generates warnings when there are too few or too many fields in the input row. The warnings are not stored anywhere; the number of warnings can only be used as an indication if everything went well. If you get warnings and want to know exactly why you got them, one way to do this is to use SELECT ... INTO OUTFILE into another file and compare this to your original input file.

If you need LOAD DATA to read from a pipe, you can use the following trick: 

mkfifo /mysql/db/x/x
chmod 666 /mysql/db/x/x
cat < /dev/tcp/10.1.1.12/4711 > /nt/mysql/db/x/x
mysql -e "LOAD DATA INFILE 'x' INTO TABLE x" x

If you are using a version of MySQL older than 3.23.25 you can only do the above with LOAD DATA LOCAL INFILE.

For more information about the efficiency of INSERT versus LOAD DATA INFILE and speeding up LOAD DATA INFILE, See section 5.2.8 Speed of INSERT Queries.

6.5 Data Definition: CREATE, DROP, ALTER

6.5.1 CREATE DATABASE Syntax

CREATE DATABASE [IF NOT EXISTS] db_name

CREATE DATABASE creates a database with the given name. Rules for allowable database names are given in section 6.1.2 Database, Table, Index, Column, and Alias Names. An error occurs if the database already exists and you didn't specify IF NOT EXISTS.

Databases in MySQL are implemented as directories containing files that correspond to tables in the database. Because there are no tables in a database when it is initially created, the CREATE DATABASE statement only creates a directory under the MySQL data directory.

You can also create databases with mysqladmin. See section 4.8 MySQL Client-Side Scripts and Utilities.

6.5.2 DROP DATABASE Syntax

DROP DATABASE [IF EXISTS] db_name

DROP DATABASE drops all tables in the database and deletes the database. If you do a DROP DATABASE on a symbolic linked database, both the link and the original database is deleted. Be VERY careful with this command!

DROP DATABASE returns the number of files that were removed from the database directory. Normally, this is three times the number of tables, because normally each table corresponds to a `.MYD' file, a `.MYI' file, and a `.frm' file.

The DROP DATABASE command removes from the given database directory all files with the following extensions:

.BAK .DAT .HSH .ISD
.ISM .ISM .MRG .MYD
.MYI .db .frm

All subdirectories that consists of 2 digits (RAID directories) are also removed.

In MySQL Version 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring if the database doesn't exist.

You can also drop databases with mysqladmin. See section 4.8 MySQL Client-Side Scripts and Utilities.

6.5.3 CREATE TABLE Syntax

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name [(create_definition,...)]
[table_options] [select_statement]

create_definition:
  col_name type [NOT NULL | NULL] [DEFAULT default_value] [AUTO_INCREMENT]
            [PRIMARY KEY] [reference_definition]
  or    PRIMARY KEY (index_col_name,...)
  or    KEY [index_name] (index_col_name,...)
  or    INDEX [index_name] (index_col_name,...)
  or    UNIQUE [INDEX] [index_name] (index_col_name,...)
  or    FULLTEXT [INDEX] [index_name] (index_col_name,...)
  or    [CONSTRAINT symbol] FOREIGN KEY index_name (index_col_name,...)
            [reference_definition]
  or    CHECK (expr)

type:
        TINYINT[(length)] [UNSIGNED] [ZEROFILL]
  or    SMALLINT[(length)] [UNSIGNED] [ZEROFILL]
  or    MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL]
  or    INT[(length)] [UNSIGNED] [ZEROFILL]
  or    INTEGER[(length)] [UNSIGNED] [ZEROFILL]
  or    BIGINT[(length)] [UNSIGNED] [ZEROFILL]
  or    REAL[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL]
  or    NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL]
  or    CHAR(length) [BINARY]
  or    VARCHAR(length) [BINARY]
  or    DATE
  or    TIME
  or    TIMESTAMP
  or    DATETIME
  or    TINYBLOB
  or    BLOB
  or    MEDIUMBLOB
  or    LONGBLOB
  or    TINYTEXT
  or    TEXT
  or    MEDIUMTEXT
  or    LONGTEXT
  or    ENUM(value1,value2,value3,...)
  or    SET(value1,value2,value3,...)

index_col_name:
        col_name [(length)]

reference_definition:
        REFERENCES tbl_name [(index_col_name,...)]
                   [MATCH FULL | MATCH PARTIAL]
                   [ON DELETE reference_option]
                   [ON UPDATE reference_option]

reference_option:
        RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

table_options:
	TYPE = {BDB | HEAP | ISAM | InnoDB | MERGE | MYISAM }
or	AUTO_INCREMENT = #
or	AVG_ROW_LENGTH = #
or	CHECKSUM = {0 | 1}
or	COMMENT = "string"
or	MAX_ROWS = #
or	MIN_ROWS = #
or	PACK_KEYS = {0 | 1}
or	PASSWORD = "string"
or	DELAY_KEY_WRITE = {0 | 1}
or      ROW_FORMAT= { default | dynamic | fixed | compressed }
or	RAID_TYPE= {1 | STRIPED | RAID0 } RAID_CHUNKS=#  RAID_CHUNKSIZE=#
or	UNION = (table_name,[table_name...])
or      DATA DIRECTORY="directory"
or      INDEX DIRECTORY="directory"

select_statement:
	[IGNORE | REPLACE] SELECT ...  (Some legal select statement)

CREATE TABLE creates a table with the given name in the current database. Rules for allowable table names are given in section 6.1.2 Database, Table, Index, Column, and Alias Names. An error occurs if there is no current database or if the table already exists.

In MySQL Version 3.22 or later, the table name can be specified as db_name.tbl_name. This works whether or not there is a current database.

In MySQL Version 3.23, you can use the TEMPORARY keyword when you create a table. A temporary table will automatically be deleted if a connection dies and the name is per connection. This means that two different connections can both use the same temporary table name without conflicting with each other or with an existing table of the same name. (The existing table is hidden until the temporary table is deleted).

In MySQL Version 3.23 or later, you can use the keywords IF NOT EXISTS so that an error does not occur if the table already exists. Note that there is no verification that the table structures are identical.

Each table tbl_name is represented by some files in the database directory. In the case of MyISAM-type tables you will get:

File Purpose
tbl_name.frm Table definition (form) file
tbl_name.MYD Data file
tbl_name.MYI Index file

For more information on the properties of the various column types, see section 6.2 Column Types:

6.5.3.1 Silent Column Specification Changes

In some cases, MySQL silently changes a column specification from that given in a CREATE TABLE statement. (This may also occur with ALTER TABLE.):

If you want to see whether or not MySQL used a column type other than the one you specified, issue a DESCRIBE tbl_name statement after creating or altering your table.

Certain other column type changes may occur if you compress a table using myisampack. See section 7.1.2.3 Compressed Table Characteristics.

6.5.4 ALTER TABLE Syntax

ALTER [IGNORE] TABLE tbl_name alter_spec [, alter_spec ...]

alter_specification:
        ADD [COLUMN] create_definition [FIRST | AFTER column_name ]
  or    ADD [COLUMN] (create_definition, create_definition,...)
  or    ADD INDEX [index_name] (index_col_name,...)
  or    ADD PRIMARY KEY (index_col_name,...)
  or    ADD UNIQUE [index_name] (index_col_name,...)
  or    ADD FULLTEXT [index_name] (index_col_name,...)
  or	ADD [CONSTRAINT symbol] FOREIGN KEY index_name (index_col_name,...)
            [reference_definition]
  or    ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  or    CHANGE [COLUMN] old_col_name create_definition
  or    MODIFY [COLUMN] create_definition
  or    DROP [COLUMN] col_name
  or    DROP PRIMARY KEY
  or    DROP INDEX index_name
  or    DISABLE KEYS
  or    ENABLE KEYS
  or    RENAME [TO] new_tbl_name
  or    ORDER BY col
  or    table_options

ALTER TABLE allows you to change the structure of an existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself. You can also change the comment for the table and type of the table. See section 6.5.3 CREATE TABLE Syntax.

If you use ALTER TABLE to change a column specification but DESCRIBE tbl_name indicates that your column was not changed, it is possible that MySQL ignored your modification for one of the reasons described in section 6.5.3.1 Silent Column Specification Changes. For example, if you try to change a VARCHAR column to CHAR, MySQL will still use VARCHAR if the table contains other variable-length columns.

ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While ALTER TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.

Note that if you use any other option to ALTER TABLE than RENAME, MySQL will always create a temporary table, even if the data wouldn't strictly need to be copied (like when you change the name of a column). We plan to fix this in the future, but as one doesn't normally do ALTER TABLE that often this isn't that high on our TODO.

Here is an example that shows some of the uses of ALTER TABLE. We begin with a table t1 that is created as shown below: 

mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from t1 to t2: 

mysql> ALTER TABLE t1 RENAME t2;

To change column a from INTEGER to TINYINT NOT NULL (leaving the name the same), and to change column b from CHAR(10) to CHAR(20) as well as renaming it from b to c: 

mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new TIMESTAMP column named d: 

mysql> ALTER TABLE t2 ADD d TIMESTAMP;

To add an index on column d, and make column a the primary key: 

mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a);

To remove column c: 

mysql> ALTER TABLE t2 DROP COLUMN c;

To add a new AUTO_INCREMENT integer column named c: 

mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
           ADD INDEX (c);

Note that we indexed c, because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because indexed columns cannot be NULL.

When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically. You can set the first sequence number by executing SET INSERT_ID=# before ALTER TABLE or using the AUTO_INCREMENT = # table option. See section 5.5.6 SET Syntax.

With MyISAM tables, if you don't change the AUTO_INCREMENT column, the sequence number will not be affected. If you drop an AUTO_INCREMENT column and then add another AUTO_INCREMENT column, the numbers will start from 1 again.

See section A.6.1 Problems with ALTER TABLE..

6.5.5 RENAME TABLE Syntax

RENAME TABLE tbl_name TO new_table_name[, tbl_name2 TO new_table_name2,...]

The rename is done atomically, which means that no other thread can access any of the tables while the rename is running. This makes it possible to replace a table with an empty one: 

CREATE TABLE new_table (...);
RENAME TABLE old_table TO backup_table, new_table TO old_table;

The rename is done from left to right, which means that if you want to swap two tables names, you have to: 

RENAME TABLE old_table    TO backup_table,
             new_table    TO old_table,
             backup_table TO new_table;

As long as two databases are on the same disk you can also rename from one database to another: 

RENAME TABLE current_database.table_name TO other_database.table_name;

When you execute RENAME, you can't have any locked tables or active transactions. You must also have the ALTER and DROP privilege on the original table and CREATE and INSERT privilege on the new table.

If MySQL encounters any errors in a multiple table rename, it will do a reverse rename for all renamed tables to get everything back to the original state.

6.5.6 DROP TABLE Syntax

DROP TABLE [IF EXISTS] tbl_name [, tbl_name,...] [RESTRICT | CASCADE]

DROP TABLE removes one or more tables. All table data and the table definition are removed, so be careful with this command!

In MySQL Version 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring for tables that don't exist.

RESTRICT and CASCADE are allowed to make porting easier. For the moment they don't do anything.

NOTE: DROP TABLE is not transaction-safe and will automatically commit any active transactions.

6.5.7 CREATE INDEX Syntax

CREATE [UNIQUE|FULLTEXT] INDEX index_name ON tbl_name (col_name[(length)],... )

The CREATE INDEX statement doesn't do anything in MySQL prior to Version 3.22. In Version 3.22 or later, CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See section 6.5.4 ALTER TABLE Syntax.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See section 6.5.3 CREATE TABLE Syntax. CREATE INDEX allows you to add indexes to existing tables.

A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

For CHAR and VARCHAR columns, indexes can be created that use only part of a column, using col_name(length) syntax. (On BLOB and TEXT columns the length is required). The statement shown below creates an index using the first 10 characters of the name column: 

mysql> CREATE INDEX part_of_name ON customer (name(10));

Because most names usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column. Also, using partial columns for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations!

Note that you can only add an index on a column that can have NULL values or on a BLOB/TEXT column if you are using MySQL Version 3.23.2 or newer and are using the MyISAM table type.

For more information about how MySQL uses indexes, see section 5.4.3 How MySQL Uses Indexes.

FULLTEXT indexes can index only VARCHAR and TEXT columns, and only in MyISAM tables. FULLTEXT indexes are available in MySQL Version 3.23.23 and later. section 6.9 MySQL Full-text Search.

6.5.8 DROP INDEX Syntax

DROP INDEX index_name ON tbl_name

DROP INDEX drops the index named index_name from the table tbl_name. DROP INDEX doesn't do anything in MySQL prior to Version 3.22. In Version 3.22 or later, DROP INDEX is mapped to an ALTER TABLE statement to drop the index. See section 6.5.4 ALTER TABLE Syntax.

6.6 Basic MySQL User Utility Commands

6.6.1 USE Syntax

USE db_name

The USE db_name statement tells MySQL to use the db_name database as the default database for subsequent queries. The database remains current until the end of the session or until another USE statement is issued: 

mysql> USE db1;
mysql> SELECT count(*) FROM mytable;      # selects from db1.mytable
mysql> USE db2;
mysql> SELECT count(*) FROM mytable;      # selects from db2.mytable

Making a particular database current by means of the USE statement does not preclude you from accessing tables in other databases. The example below accesses the author table from the db1 database and the editor table from the db2 database: 

mysql> USE db1;
mysql> SELECT author_name,editor_name FROM author,db2.editor
           WHERE author.editor_id = db2.editor.editor_id;

The USE statement is provided for Sybase compatibility.

6.6.2 DESCRIBE Syntax (Get Information About Columns)

{DESCRIBE | DESC} tbl_name {col_name | wild}

DESCRIBE is a shortcut for SHOW COLUMNS FROM. See section 4.5.5.1 Retrieving information about Database, Tables, Columns, and Indexes.

DESCRIBE provides information about a table's columns. col_name may be a column name or a string containing the SQL `%' and `_' wild-card characters.

If the column types are different than you expect them to be based on a CREATE TABLE statement, note that MySQL sometimes changes column types. See section 6.5.3.1 Silent Column Specification Changes.

This statement is provided for Oracle compatibility.

The SHOW statement provides similar information. See section 4.5.5 SHOW Syntax.

6.7 MySQL Transactional and Locking Commands

6.7.1 BEGIN/COMMIT/ROLLBACK Syntax

By default, MySQL runs in autocommit mode. This means that as soon as you execute an update, MySQL will store the update on disk.

If you are using transactions safe tables (like BDB, InnoDB, you can put MySQL into non-autocommit mode with the following command: 

SET AUTOCOMMIT=0

After this you must use COMMIT to store your changes to disk or ROLLBACK if you want to ignore the changes you have made since the beginning of your transaction.

If you want to switch from AUTOCOMMIT mode for one series of statements, you can use the BEGIN or BEGIN WORK statement: 

BEGIN;
SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summmary=@A WHERE type=1;
COMMIT;

Note that if you are using non-transaction-safe tables, the changes will be stored at once, independent of the status of the autocommit mode.

If you do a ROLLBACK when you have updated a non-transactional table you will get an error (ER_WARNING_NOT_COMPLETE_ROLLBACK) as a warning. All transactional safe tables will be restored but any non-transactional table will not change.

If you are using BEGIN or SET AUTOCOMMIT=0, you should use the MySQL binary log for backups instead of the older update log. Transactions are stored in the binary log in one chunk, upon COMMIT, to ensure that transactions which are rolled back are not stored. See section 4.9.4 The Binary Update Log.

The following commands automatically end a transaction (as if you had done a COMMIT before executing the command):

ALTER TABLE BEGIN CREATE INDEX
DROP DATABASE DROP TABLE RENAME TABLE
TRUNCATE

You can change the isolation level for transactions with SET TRANSACTION ISOLATION LEVEL .... See section 6.7.3 SET TRANSACTION Syntax.

6.7.2 LOCK TABLES/UNLOCK TABLES Syntax

LOCK TABLES tbl_name [AS alias] {READ | [READ LOCAL] | [LOW_PRIORITY] WRITE}
            [, tbl_name {READ | [LOW_PRIORITY] WRITE} ...]
...
UNLOCK TABLES

LOCK TABLES locks tables for the current thread. UNLOCK TABLES releases any locks held by the current thread. All tables that are locked by the current thread are automatically unlocked when the thread issues another LOCK TABLES, or when the connection to the server is closed.

The main reasons to use LOCK TABLES are for emulating transactions or getting more speed when updating tables. This is explained in more detail later.

If a thread obtains a READ lock on a table, that thread (and all other threads) can only read from the table. If a thread obtains a WRITE lock on a table, then only the thread holding the lock can READ from or WRITE to the table. Other threads are blocked.

The difference between READ LOCAL and READ is that READ LOCAL allows non-conflicting INSERT statements to execute while the lock is held. This can't however be used if you are going to manipulate the database files outside MySQL while you hold the lock.

When you use LOCK TABLES, you must lock all tables that you are going to use and you must use the same alias that you are going to use in your queries! If you are using a table multiple times in a query (with aliases), you must get a lock for each alias!

WRITE locks normally have higher priority than READ locks, to ensure that updates are processed as soon as possible. This means that if one thread obtains a READ lock and then another thread requests a WRITE lock, subsequent READ lock requests will wait until the WRITE thread has gotten the lock and released it. You can use LOW_PRIORITY WRITE locks to allow other threads to obtain READ locks while the thread is waiting for the WRITE lock. You should only use LOW_PRIORITY WRITE locks if you are sure that there will eventually be a time when no threads will have a READ lock.

LOCK TABLES works as follows:

  1. Sort all tables to be locked in a internally defined order (from the user standpoint the order is undefined).
  2. If a table is locked with a read and a write lock, put the write lock before the read lock.
  3. Lock one table at a time until the thread gets all locks.

This policy ensures that table locking is deadlock free. There is however other things one needs to be aware of with this schema:

If you are using a LOW_PRIORITY_WRITE lock for a table, this means only that MySQL will wait for this particlar lock until there is no threads that wants a READ lock. When the thread has got the WRITE lock and is waiting to get the lock for the next table in the lock table list, all other threads will wait for the WRITE lock to be released. If this becomes a serious problem with your application, you should consider converting some of your tables to transactions safe tables.

You can safely kill a thread that is waiting for a table lock with KILL. See section 4.5.4 KILL Syntax.

Note that you should NOT lock any tables that you are using with INSERT DELAYED. This is because that in this case the INSERT is done by a separate thread.

Normally, you don't have to lock tables, as all single UPDATE statements are atomic; no other thread can interfere with any other currently executing SQL statement. There are a few cases when you would like to lock tables anyway:

By using incremental updates (UPDATE customer SET value=value+new_value) or the LAST_INSERT_ID() function, you can avoid using LOCK TABLES in many cases.

You can also solve some cases by using the user-level lock functions GET_LOCK() and RELEASE_LOCK(). These locks are saved in a hash table in the server and implemented with pthread_mutex_lock() and pthread_mutex_unlock() for high speed. See section 6.3.5.2 Miscellaneous Functions.

See section 5.3.1 How MySQL Locks Tables, for more information on locking policy.

You can lock all tables in all databases with read locks with the FLUSH TABLES WITH READ LOCK command. See section 4.5.3 FLUSH Syntax. This is very convenient way to get backups if you have a file system, like Veritas, that can take snapshots in time.

NOTE: LOCK TABLES is not transaction-safe and will automatically commit any active transactions before attempting to lock the tables.

6.7.3 SET TRANSACTION Syntax

SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
[READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE]

Sets the transaction isolation level for the global, whole session or the next transaction.

The default behavior is to set the isolation level for the next (not started) transaction.

If you set the GLOBAL privilege it will affect all new created threads. You will need the PROCESS privilege to do do this.

Setting the SESSION privilege will affect the following and all future transactions.

You can set the default isolation level for mysqld with --transaction-isolation=.... See section 4.1.1 mysqld Command-line Options.

6.8 HANDLER Syntax

HANDLER table OPEN [ AS alias ]
HANDLER table READ index { = | >= | <= | < } (value1, value2, ... )  [ WHERE ... ] [LIMIT ... ]
HANDLER table READ index { FIRST | NEXT | PREV | LAST } [ WHERE ... ] [LIMIT ... ]
HANDLER table READ { FIRST | NEXT }  [ WHERE ... ] [LIMIT ... ]
HANDLER table CLOSE

The HANDLER statement provides direct access to MySQL table interface, bypassing SQL optimizer. Thus, it is faster then SELECT.

The first form of HANDLER statement opens a table, making in accessible via the following HANDLER ... READ routines.

The second form fetches one (or, specified by LIMIT clause) row where the index specified complies to the condition and WHERE condition is met. If the index consists of several parts (spans over several columns) the values are specified in comma-separated list, providing values only for few first columns is possible.

The third form fetches one (or, specified by LIMIT clause) row from the table in index order, matching WHERE condition.

The fourth form (without index specification) fetches one (or, specified by LIMIT clause) row from the table in natural row order (as stored in data file) matching WHERE condition. It is faster than HANDLER table READ index when full table scan is desired.

The last form closes the table, opened with HANDLER ... OPEN.

HANDLER is somewhat low-level statement, for example it does not provide consistency. That is HANDLER ... OPEN does not takes a snapshot of the table, and does not locks the table. The above means, that after HANDLER ... OPEN table data can be modified (by this or other thread) and these modifications may appear only partially in HANDLER ... NEXT or HANDLER ... PREV scans.

6.9 MySQL Full-text Search

Since Version 3.23.23, MySQL has support for full-text indexing and searching. Full-text indexes in MySQL are an index of type FULLTEXT. FULLTEXT indexes can be created from VARCHAR and TEXT columns at CREATE TABLE time or added later with ALTER TABLE or CREATE INDEX. For large datasets, adding FULLTEXT index with ALTER TABLE (or CREATE INDEX) would be much faster than inserting rows into the empty table with a FULLTEXT index.

Full-text search is performed with the MATCH function. 

mysql> CREATE TABLE articles (
    ->   id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY, 
    ->   title VARCHAR(200),
    ->   body TEXT,
    ->   FULLTEXT (title,body)
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO articles VALUES
    -> (0,'MySQL Tutorial', 'DBMS stands for DataBase Management ...'),
    -> (0,'How To Use MySQL Efficiently', 'After you went through a ...'),
    -> (0,'Optimizing MySQL','In this tutorial we will show how to ...'),
    -> (0,'1001 MySQL Trick','1. Never run mysqld as root. 2. Normalize ...'),
    -> (0,'MySQL vs. YourSQL', 'In the following database comparison we ...'),
    -> (0,'MySQL Security', 'When configured properly, MySQL could be ...');
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM articles WHERE MATCH (title,body) AGAINST ('database');
+----+-------------------+---------------------------------------------+
| id | title             | body                                        |
+----+-------------------+---------------------------------------------+
|  5 | MySQL vs. YourSQL | In the following database comparison we ... |
|  1 | MySQL Tutorial    | DBMS stands for DataBase Management ...     |
+----+-------------------+---------------------------------------------+
2 rows in set (0.00 sec)

The function MATCH matches a natural language query AGAINST a text collection (which is simply the set of columns covered by a FULLTEXT index). For every row in a table it returns relevance - a similarity measure between the text in that row (in the columns that are part of the collection) and the query. When it is used in a WHERE clause (see example above) the rows returned are automatically sorted with relevance decreasing. Relevance is a non-negative floating-point number. Zero relevance means no similarity. Relevance is computed based on the number of words in the row, the number of unique words in that row, the total number of words in the collection, and the number of documents (rows) that contain a particular word.

The above is a basic example of using MATCH function. Rows are returned with relevance decreasing. 

mysql> SELECT id,MATCH (title,body) AGAINST ('Tutorial') FROM articles;
+----+-----------------------------------------+
| id | MATCH (title,body) AGAINST ('Tutorial') |
+----+-----------------------------------------+
|  1 |                        0.64840710366884 |
|  2 |                                       0 |
|  3 |                        0.66266459031789 |
|  4 |                                       0 |
|  5 |                                       0 |
|  6 |                                       0 |
+----+-----------------------------------------+
5 rows in set (0.00 sec)

This example shows how to retrieve the relevances. As neither WHERE nor ORDER BY clauses are present, returned rows are not ordered. 

mysql> SELECT id, body, MATCH (title,body) AGAINST (
    -> 'Security implications of running MySQL as root') AS score
    -> FROM articles WHERE MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root');
+----+-----------------------------------------------+-----------------+
| id | body                                          | score           |
+----+-----------------------------------------------+-----------------+
|  4 | 1. Never run mysqld as root. 2. Normalize ... | 1.5055546709332 |
|  6 | When configured properly, MySQL could be ...  |   1.31140957288 |
+----+-----------------------------------------------+-----------------+
2 rows in set (0.00 sec)

This is more complex example - the query returns the relevance and still sorts the rows with relevance decreasing. To achieve it one should specify MATCH twice. Note, that this will cause no additional overhead, as MySQL optimizer will notice that these two MATCH calls are identical and will call full-text search code only once.

MySQL uses a very simple parser to split text into words. A ``word'' is any sequence of letters, numbers, `'', and `_'. Any ``word'' that is present in the stopword list or just too short (3 characters or less) is ignored.

Every correct word in the collection and in the query is weighted, according to its significance in the query or collection. This way, a word that is present in many documents will have lower weight (and may even have a zero weight), because it has lower semantic value in this particular collection. Otherwise, if the word is rare, it will receive a higher weight. The weights of the words are then combined to compute the relevance of the row.

Such a technique works best with large collections (in fact, it was carefully tuned this way). For very small tables, word distribution does not reflect adequately their semantical value, and this model may sometimes produce bizarre results. 

mysql> SELECT * FROM articles WHERE MATCH (title,body) AGAINST ('MySQL');
Empty set (0.00 sec)

Search for the word MySQL produces no results in the above example. Word MySQL is present in more than half of rows, and as such, is effectively treated as a stopword (that is, with semantical value zero). It is, really, the desired behavior - a natural language query should not return every second row in 1GB table.

A word that matches half of rows in a table is less likely to locate relevant documents. In fact, it will most likely find plenty of irrelevant documents. We all know this happens far too often when we are trying to find something on the Internet with a search engine. It is with this reasoning that such rows have been assigned a low semantical value in this particular dataset.

6.9.1 Fulltext restrictions

6.9.2 Fine-tuning MySQL Full-text Search

Unfortunately, full-text search has no user-tunable parameters yet, although adding some is very high on the TODO. However, if you have a MySQL source distribution (See section 2.3 Installing a MySQL Source Distribution.), you can somewhat alter the full-text search behavior.

Note that full-text search was carefully tuned for the best searching effectiveness. Modifying the default behavior will, in most cases, only make the search results worse. Do not alter the MySQL sources unless you know what you are doing!

6.9.3 New Features of Full-text Search to Appear in MySQL 4.0

This section includes a list of the fulltext features that are already implemented in the 4.0 tree. It explains More functions for full-text search entry of section 1.6.1 Things that should be in 4.0.

6.9.4 Full-text Search TODO

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