MySQL Server supports some extensions that you probably will not find in other SQL DBMSs. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the following form:
/*! MySQL-specific code */
In this case, MySQL Server parses and executes the code within
the comment as it would any other SQL statement, but other SQL
servers will ignore the extensions. For example, MySQL Server
recognizes the STRAIGHT_JOIN
keyword in the
following statement, but other servers will not:
SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ...
If you add a version number after the !
character, the syntax within the comment is executed only if the
MySQL version is greater than or equal to the specified version
number. The KEY_BLOCK_SIZE
clause in the
following comment is executed only by servers from MySQL 5.1.10
or higher:
CREATE TABLE t1(a INT, KEY (a)) /*!50110 KEY_BLOCK_SIZE=1024 */;
The following descriptions list MySQL extensions, organized by category.
Organization of data on disk
MySQL Server maps each database to a directory under the MySQL data directory, and maps tables within a database to file names in the database directory. Consequently, database and table names are case-sensitive in MySQL Server on operating systems that have case-sensitive file names (such as most Unix systems). See Section 9.2.2, “Identifier Case Sensitivity”.
General language syntax
By default, strings can be enclosed by
"
as well as'
. If theANSI_QUOTES
SQL mode is enabled, strings can be enclosed only by'
and the server interprets strings enclosed by"
as identifiers.\
is the escape character in strings.In SQL statements, you can access tables from different databases with the
db_name.tbl_name
syntax. Some SQL servers provide the same functionality but call thisUser space
. MySQL Server doesn't support tablespaces such as used in statements like this:CREATE TABLE ralph.my_table ... IN my_tablespace
.
SQL statement syntax
The
ANALYZE TABLE
,CHECK TABLE
,OPTIMIZE TABLE
, andREPAIR TABLE
statements.The
CREATE DATABASE
,DROP DATABASE
, andALTER DATABASE
statements. See Section 13.1.12, “CREATE DATABASE Syntax”, Section 13.1.24, “DROP DATABASE Syntax”, and Section 13.1.2, “ALTER DATABASE Syntax”.The
DO
statement.EXPLAIN SELECT
to obtain a description of how tables are processed by the query optimizer.The
SET
statement. See Section 13.7.5.1, “SET Syntax for Variable Assignment”.The
SHOW
statement. See Section 13.7.6, “SHOW Syntax”. The information produced by many of the MySQL-specificSHOW
statements can be obtained in more standard fashion by usingSELECT
to queryINFORMATION_SCHEMA
. See Chapter 25, INFORMATION_SCHEMA Tables.Use of
LOAD DATA
. In many cases, this syntax is compatible with OracleLOAD DATA
. See Section 13.2.7, “LOAD DATA Syntax”.Use of
RENAME TABLE
. See Section 13.1.36, “RENAME TABLE Syntax”.Use of
REPLACE
instead ofDELETE
plusINSERT
. See Section 13.2.9, “REPLACE Syntax”.Use of
CHANGE
,col_name
DROP
, orcol_name
DROP INDEX
,IGNORE
orRENAME
inALTER TABLE
statements. Use of multipleADD
,ALTER
,DROP
, orCHANGE
clauses in anALTER TABLE
statement. See Section 13.1.9, “ALTER TABLE Syntax”.Use of index names, indexes on a prefix of a column, and use of
INDEX
orKEY
inCREATE TABLE
statements. See Section 13.1.20, “CREATE TABLE Syntax”.Use of
TEMPORARY
orIF NOT EXISTS
withCREATE TABLE
.Use of
IF EXISTS
withDROP TABLE
andDROP DATABASE
.The capability of dropping multiple tables with a single
DROP TABLE
statement.The
ORDER BY
andLIMIT
clauses of theUPDATE
andDELETE
statements.INSERT INTO
syntax.tbl_name
SETcol_name
= ...The
LOW_PRIORITY
clause of theINSERT
,REPLACE
,DELETE
, andUPDATE
statements.Use of
INTO OUTFILE
orINTO DUMPFILE
inSELECT
statements. See Section 13.2.10, “SELECT Syntax”.Options such as
STRAIGHT_JOIN
orSQL_SMALL_RESULT
inSELECT
statements.You don't need to name all selected columns in the
GROUP BY
clause. This gives better performance for some very specific, but quite normal queries. See Section 12.20, “Aggregate (GROUP BY) Functions”.You can specify
ASC
andDESC
withGROUP BY
, not just withORDER BY
.The ability to set variables in a statement with the
:=
assignment operator. See Section 9.4, “User-Defined Variables”.
Data types
Functions and operators
To make it easier for users who migrate from other SQL environments, MySQL Server supports aliases for many functions. For example, all string functions support both standard SQL syntax and ODBC syntax.
MySQL Server understands the
||
and&&
operators to mean logical OR and AND, as in the C programming language. In MySQL Server,||
andOR
are synonyms, as are&&
andAND
. Because of this nice syntax, MySQL Server doesn't support the standard SQL||
operator for string concatenation; useCONCAT()
instead. BecauseCONCAT()
takes any number of arguments, it is easy to convert use of the||
operator to MySQL Server.Use of
COUNT(DISTINCT
wherevalue_list
)value_list
has more than one element.String comparisons are case insensitive by default, with sort ordering determined by the collation of the current character set, which is
utf8mb4
by default. To perform case-sensitive comparisons instead, you should declare your columns with theBINARY
attribute or use theBINARY
cast, which causes comparisons to be done using the underlying character code values rather than a lexical ordering.The
%
operator is a synonym forMOD()
. That is,
is equivalent toN
%M
MOD(
.N
,M
)%
is supported for C programmers and for compatibility with PostgreSQL.The
=
,<>
,<=
,<
,>=
,>
,<<
,>>
,<=>
,AND
,OR
, orLIKE
operators may be used in expressions in the output column list (to the left of theFROM
) inSELECT
statements. For example:mysql> SELECT col1=1 AND col2=2 FROM my_table;
The
LAST_INSERT_ID()
function returns the most recentAUTO_INCREMENT
value. See Section 12.15, “Information Functions”.LIKE
is permitted on numeric values.The
REGEXP
andNOT REGEXP
extended regular expression operators.CONCAT()
orCHAR()
with one argument or more than two arguments. (In MySQL Server, these functions can take a variable number of arguments.)The
BIT_COUNT()
,CASE
,ELT()
,FROM_DAYS()
,FORMAT()
,IF()
,MD5()
,PERIOD_ADD()
,PERIOD_DIFF()
,TO_DAYS()
, andWEEKDAY()
functions.Use of
TRIM()
to trim substrings. Standard SQL supports removal of single characters only.The
GROUP BY
functionsSTD()
,BIT_OR()
,BIT_AND()
,BIT_XOR()
, andGROUP_CONCAT()
. See Section 12.20, “Aggregate (GROUP BY) Functions”.