MySQL Server supports some extensions that you probably won't find in other SQL DBMSs. Be warned that if you use them, your code won't 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:
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_JOINkeyword 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
TEMPORARYkeyword in the following comment is executed only by servers from MySQL 3.23.02 or higher:
CREATE /*!32302 TEMPORARY */ TABLE t (a INT);
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. This has a few implications:
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 8.2.2, “Identifier Case Sensitivity”.
You can use standard system commands to back up, rename, move, delete, and copy tables that are managed by the
MyISAMstorage engine. For example, it is possible to rename a
MyISAMtable by renaming the
.frmfiles to which the table corresponds. (Nevertheless, it is preferable to use
ALTER TABLE ... RENAMEand let the server rename the files.)
Prior to MySQL 5.1.6, database and table names cannot contain path name separator characters (“
General language syntax
By default, strings can be enclosed by either “
"” or “
'”, not just by “
'”. (If the
ANSI_QUOTESSQL 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_namesyntax. Some SQL servers provide the same functionality but call this
User 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
EXPLAIN SELECTto obtain a description of how tables are processed by the query optimizer.
SHOWstatement. See Section 12.4.5, “
SHOWSyntax”. The information produced by many of the MySQL-specific
SHOWstatements can be obtained in more standard fashion by using
INFORMATION_SCHEMA. See Chapter 20,
ALTER TABLEstatements. Use of multiple
CHANGEclauses in an
ALTER TABLEstatement. See Section 12.1.7, “
IF NOT EXISTSwith
The capability of dropping multiple tables with a single
Options such as
You don't need to name all selected columns in the
GROUP BYclause. This gives better performance for some very specific, but quite normal queries. See Section 11.12, “Functions and Modifiers for Use with
You can specify
GROUP BY, not just with
The ability to set variables in a statement with the
SELECT @a:=SUM(total),@b:=COUNT(*),@a/@b AS avg->
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
&&operators to mean logical OR and AND, as in the C programming language. In MySQL Server,
ORare synonyms, as are
AND. Because of this nice syntax, MySQL Server doesn't support the standard SQL
||operator for string concatenation; use
CONCAT()takes any number of arguments, it is easy to convert use of the
||operator to MySQL Server.
value_listhas 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
latin1(cp1252 West European) by default. If you don't like this, you should declare your columns with the
BINARYattribute or use the
BINARYcast, which causes comparisons to be done using the underlying character code values rather then a lexical ordering.
SELECT col1=1 AND col2=2 FROM my_table;
LIKEis allowed on numeric values.
TRIM()to trim substrings. Standard SQL supports removal of single characters only.