Certain objects within MySQL, including database, table, index, column, alias, view, stored procedure, partition, tablespace, and other object names are known as identifiers. This section describes the allowable syntax for identifiers in MySQL. Section 8.2.2, “Identifier Case Sensitivity”, describes which types of identifiers are case sensitive and under what conditions.
An identifier may be quoted or unquoted. If an identifier contains special characters or is a reserved word, you must quote it whenever you refer to it. The set of alphanumeric characters from the current character set, “
_”, and “
$” are not special. Reserved words are listed at Section 8.3, “Reserved Words”. (Exception: A reserved word that follows a period in a qualified name must be an identifier, so it need not be quoted.)
The identifier quote character is the backtick (“
SELECT * FROM `select` WHERE `select`.id > 100;
ANSI_QUOTESSQL mode is enabled, it is also allowable to quote identifiers within double quotation marks:
CREATE TABLE "test" (col INT);ERROR 1064: You have an error in your SQL syntax... mysql>
CREATE TABLE "test" (col INT);Query OK, 0 rows affected (0.00 sec)
ANSI_QUOTESmode causes the server to interpret double-quoted strings as identifiers. Consequently, when this mode is enabled, string literals must be enclosed within single quotation marks. They cannot be enclosed within double quotation marks. The server SQL mode is controlled as described in Section 5.1.7, “Server SQL Modes”.
Identifier quote characters can be included within an identifier if you quote the identifier. If the character to be included within the identifier is the same as that used to quote the identifier itself, then you need to double the character. The following statement creates a table named
a`bthat contains a column named
CREATE TABLE `a``b` (`c"d` INT);
In the select list of a query, a quoted column alias can be specified using identifier or string quoting characters:
SELECT 1 AS `one`, 2 AS 'two';+-----+-----+ | one | two | +-----+-----+ | 1 | 2 | +-----+-----+
Elsewhere in the statement, quoted references to the alias must use identifier quoting or the reference is treated as a string literal.
Identifiers may begin with a digit but unless quoted may not consist solely of digits.
It is recommended that you do not use names that begin with
Nare integers. For example, avoid using
1eas an identifier, because an expression such as
1e+3is ambiguous. Depending on context, it might be interpreted as the expression
1e + 3or as the number
Be careful when using
MD5()to produce table names because it can produce names in illegal or ambiguous formats such as those just described.
A user variable cannot be used directly in an SQL statement as an identifier or as part of an identifier. See Section 8.4, “User-Defined Variables”, for more information and examples of workarounds.
There are some restrictions on the characters that may appear in identifiers:
No identifier can contain ASCII NUL (
Database, table, and column names should not end with space characters.
Before MySQL 5.1.6, database and table names cannot contain “
.”, or characters that are not allowed in file names.
As of MySQL 5.1.6, special characters in database and table names are encoded in the corresponding file system names as described in Section 8.2.3, “Mapping of Identifiers to File Names”. If you have databases or tables from an older version of MySQL that contain special characters and for which the underlying directory names or file names have not been updated to use the new encoding, the server displays their names with a prefix of
#mysql50#. For information about referring to such names or converting them to the newer encoding, see that section.
The following table describes the maximum length for each type of identifier.
Identifier Maximum Length (characters) Database 64 Table 64 Column 64 Index 64 Constraint 64 Stored Procedure or Function 64 Trigger 64 View 64 Event 64 Tablespace 64 Log File Group 64 Alias 256 (see exception following table) Compound Statement Label 16
As of MySQL 5.1.23, aliases for column names in
CREATE VIEWstatements are checked against the maximum column length of 64 characters (not the maximum alias length of 256 characters).
Identifiers are stored using Unicode (UTF-8). This applies to identifiers in table definitions that are stored in
.frmfiles and to identifiers stored in the grant tables in the
mysqldatabase. The sizes of the identifier string columns in the grant tables are measured in characters. You can use multi-byte characters without reducing the number of characters allowed for values stored in these columns, something not true prior to MySQL 4.1. The allowable Unicode characters are those in the Basic Multilingual Plane (BMP). Supplementary characters are not allowed.