Database, table, index, column, and alias names are 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 (“`”):

mysql> SELECT * FROM `select` WHERE `select`.id > 100;

If the ANSI_QUOTES SQL mode is enabled, it is also allowable to quote identifiers within double quotes:

mysql> CREATE TABLE "test" (col INT);
ERROR 1064: You have an error in your SQL syntax...
mysql> SET sql_mode='ANSI_QUOTES';
mysql> CREATE TABLE "test" (col INT);
Query OK, 0 rows affected (0.00 sec)

The ANSI_QUOTES mode causes the server to interpret double-quoted strings as identifiers. Consequently, when this mode is enabled, string literals must be enclosed within single quotes. They cannot be enclosed within double quotes. The server SQL mode is controlled as described in Section 5.1.7, “Server SQL Modes”.

As of MySQL 4.1, 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, double the character. The following statement creates a table named a`b that contains a column named c"d:

mysql> 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:

mysql> SELECT 1 AS `one`, 2 AS 'two';
+-----+-----+
| one | two |
+-----+-----+
|   1 |   2 |
+-----+-----+

Identifier quoting was introduced in MySQL 3.23.6 to allow use of identifiers that contain special characters or are reserved words. Before 3.23.6, you cannot use identifiers that require quotes, so the only legal characters are the set of alphanumeric characters from the current character set, “_”, and “$”.

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 Me or MeN, where M and N are integers. For example, avoid using 1e as an identifier, because an expression such as 1e+3 is ambiguous. Depending on context, it might be interpreted as the expression 1e + 3 or as the number 1e+3.

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:

The following table describes the maximum length for each type of identifier. Before MySQL 4.1.5, the maximum-length restrictions on identifiers are measured in bytes, not characters. Until that version, if you use multi-byte characters in your identifier names, the maximum length will depend on the byte count of all the characters used.

Beginning with MySQL 4.1, identifiers are stored using Unicode (UTF-8). This applies to identifiers in table definitions that are stored in .frm files and to identifiers stored in the grant tables in the mysql database. 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.