10.3.6 Character String Literal Character Set and Collation

Every character string literal has a character set and a collation.

For the simple statement SELECT 'string', the string has the connection default character set and collation defined by the character_set_connection and collation_connection system variables.

A character string literal may have an optional character set introducer and COLLATE clause, to designate it as a string that uses a particular character set and collation:

[_charset_name]'string' [COLLATE collation_name]

The _charset_name expression is formally called an introducer. It tells the parser, “the string that follows uses character set charset_name.” An introducer does not change the string to the introducer character set like CONVERT() would do. It does not change the string value, although padding may occur. The introducer is just a signal. See Section 10.3.8, “Character Set Introducers”.

Examples:

SELECT 'abc';
SELECT _latin1'abc';
SELECT _binary'abc';
SELECT _utf8'abc' COLLATE utf8_danish_ci;

Character set introducers and the COLLATE clause are implemented according to standard SQL specifications.

MySQL determines the character set and collation of a character string literal in the following manner:

• If both _charset_name and COLLATE collation_name are specified, character set charset_name and collation collation_name are used. collation_name must be a permitted collation for charset_name.

• If _charset_name is specified but COLLATE is not specified, character set charset_name and its default collation are used. To see the default collation for each character set, use the SHOW CHARACTER SET statement or query the INFORMATION_SCHEMA CHARACTER_SETS table.

• If _charset_name is not specified but COLLATE collation_name is specified, the connection default character set given by the character_set_connection system variable and collation collation_name are used. collation_name must be a permitted collation for the connection default character set.

• Otherwise (neither _charset_name nor COLLATE collation_name is specified), the connection default character set and collation given by the character_set_connection and collation_connection system variables are used.

Examples:

• A nonbinary string with latin1 character set and latin1_german1_ci collation:

  SELECT _latin1'Müller' COLLATE latin1_german1_ci;

• A nonbinary string with utf8 character set and its default collation (that is, utf8_general_ci):

  SELECT _utf8'Müller';

• A binary string with binary character set and its default collation (that is, binary):

  SELECT _binary'Müller';

• A nonbinary string with the connection default character set and utf8_general_ci collation (fails if the connection character set is not utf8):

  SELECT 'Müller' COLLATE utf8_general_ci;

• A string with the connection default character set and collation:

  SELECT 'Müller';

An introducer indicates the character set for the following string, but does not change how the parser performs escape processing within the string. Escapes are always interpreted by the parser according to the character set given by character_set_connection.

The following examples show that escape processing occurs using character_set_connection even in the presence of an introducer. The examples use SET NAMES (which changes character_set_connection, as discussed in Section 10.4, “Connection Character Sets and Collations”), and display the resulting strings using the HEX() function so that the exact string contents can be seen.

Example 1:

mysql> SET NAMES latin1;
mysql> SELECT HEX('à\n'), HEX(_sjis'à\n');
+------------+-----------------+
| HEX('à\n')  | HEX(_sjis'à\n')  |
+------------+-----------------+
| E00A       | E00A            |
+------------+-----------------+

Here, à (hexadecimal value E0) is followed by \n, the escape sequence for newline. The escape sequence is interpreted using the character_set_connection value of latin1 to produce a literal newline (hexadecimal value 0A). This happens even for the second string. That is, the _sjis introducer does not affect the parser's escape processing.

Example 2:

mysql> SET NAMES sjis;
mysql> SELECT HEX('à\n'), HEX(_latin1'à\n');
+------------+-------------------+
| HEX('à\n')  | HEX(_latin1'à\n')  |
+------------+-------------------+
| E05C6E     | E05C6E            |
+------------+-------------------+

Here, character_set_connection is sjis, a character set in which the sequence of à followed by \ (hexadecimal values 05 and 5C) is a valid multibyte character. Hence, the first two bytes of the string are interpreted as a single sjis character, and the \ is not interpreted as an escape character. The following n (hexadecimal value 6E) is not interpreted as part of an escape sequence. This is true even for the second string; the _latin1 introducer does not affect escape processing.