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10.3. Specifying Character Sets and Collations

There are default settings for character sets and collations at four levels: server, database, table, and column. The following description may appear complex, but it has been found in practice that multiple-level defaulting leads to natural and obvious results.

CHARACTER SET is used in clauses that specify a character set. CHARSET may be used as a synonym for CHARACTER SET.

10.3.1. Server Character Set and Collation

MySQL Server has a server character set and a server collation. These can be set at server startup and changed at runtime.

Initially, the server character set and collation depend on the options that you use when you start mysqld. You can use --character-set-server for the character set. Along with it, you can add --collation-server for the collation. If you don't specify a character set, that is the same as saying --character-set-server=latin1. If you specify only a character set (for example, latin1) but not a collation, that is the same as saying --character-set-server=latin1 --collation-server=latin1_swedish_ci because latin1_swedish_ci is the default collation for latin1. Therefore, the following three commands all have the same effect:

shell> mysqld
shell> mysqld --character-set-server=latin1
shell> mysqld --character-set-server=latin1 \
           --collation-server=latin1_swedish_ci

One way to change the settings is by recompiling. If you want to change the default server character set and collation when building from sources, use: --with-charset and --with-collation as arguments for configure. For example:

shell> ./configure --with-charset=utf-8

Or:

shell> ./configure --with-charset=utf-8 \
           --with-collation=latin1_german1_ci

Both mysqld and configure verify that the character set/collation combination is valid. If not, each program displays an error message and terminates.

The current server character set and collation can be determined from the values of the character_set_server and collation_server system variables. These variables can be changed at runtime.

10.3.2. Database Character Set and Collation

Every database has a database character set and a database collation. The CREATE DATABASE and ALTER DATABASE statements have optional clauses for specifying the database character set and collation:

CREATE DATABASE db_name
    [[DEFAULT] CHARACTER SET charset_name]
    [[DEFAULT] COLLATE collation_name]

ALTER DATABASE db_name
    [[DEFAULT] CHARACTER SET charset_name]
    [[DEFAULT] COLLATE collation_name]

The keyword SCHEMA can be used instead of DATABASE.

All database options are stored in a text file named db.opt that can be found in the database directory.

The CHARACTER SET and COLLATE clauses make it possible to create databases with different character sets and collations on the same MySQL server.

Example:

CREATE DATABASE db_name CHARACTER SET latin1 COLLATE latin1_swedish_ci;

MySQL chooses the database character set and database collation in the following manner:

  • If both CHARACTER SET X and COLLATE Y were specified, then character set X and collation Y.

  • If CHARACTER SET X was specified without COLLATE, then character set X and its default collation.

  • If COLLATE Y was specified without CHARACTER SET, then the character set associated with Y and collation Y.

  • Otherwise, the server character set and server collation.

The database character set and collation are used as default values if the table character set and collation are not specified in CREATE TABLE statements. They have no other purpose.

The character set and collation for the default database can be determined from the values of the character_set_database and collation_database system variables. The server sets these variables whenever the default database changes. If there is no default database, the variables have the same value as the corresponding server-level system variables, character_set_server and collation_server.

10.3.3. Table Character Set and Collation

Every table has a table character set and a table collation. The CREATE TABLE and ALTER TABLE statements have optional clauses for specifying the table character set and collation:

CREATE TABLE tbl_name (column_list)
    [[DEFAULT] CHARACTER SET charset_name] [COLLATE collation_name]]

ALTER TABLE tbl_name
    [[DEFAULT] CHARACTER SET charset_name] [COLLATE collation_name]

Example:

CREATE TABLE t1 ( ... ) CHARACTER SET latin1 COLLATE latin1_danish_ci;

MySQL chooses the table character set and collation in the following manner:

  • If both CHARACTER SET X and COLLATE Y were specified, then character set X and collation Y.

  • If CHARACTER SET X was specified without COLLATE, then character set X and its default collation.

  • If COLLATE Y was specified without CHARACTER SET, then the character set associated with Y and collation Y.

  • Otherwise, the database character set and collation.

The table character set and collation are used as default values if the column character set and collation are not specified in individual column definitions. The table character set and collation are MySQL extensions; there are no such things in standard SQL.

10.3.4. Column Character Set and Collation

Every “character” column (that is, a column of type CHAR, VARCHAR, or TEXT) has a column character set and a column collation. Column definition syntax has optional clauses for specifying the column character set and collation:

col_name {CHAR | VARCHAR | TEXT} (col_length) [CHARACTER SET charset_name] [COLLATE collation_name]

Example:

CREATE TABLE Table1
(
    column1 VARCHAR(5) CHARACTER SET latin1 COLLATE latin1_german1_ci
);

MySQL chooses the column character set and collation in the following manner:

  • If both CHARACTER SET X and COLLATE Y were specified, then character set X and collation Y are used.

  • If CHARACTER SET X was specified without COLLATE, then character set X and its default collation are used.

  • If COLLATE Y was specified without CHARACTER SET, then the character set associated with Y and collation Y.

  • Otherwise, the table character set and collation are used.

The CHARACTER SET and COLLATE clauses are standard SQL.

10.3.5. Character String Literal Character Set and Collation

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

A character string literal may have an optional character set introducer and COLLATE clause:

[_charset_name]'string' [COLLATE collation_name]

Examples:

SELECT 'string';
SELECT _latin1'string';
SELECT _latin1'string' COLLATE latin1_danish_ci;

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

The _charset_name expression is formally called an introducer. It tells the parser, “the string that is about to follow uses character set X.” Because this has confused people in the past, we emphasize that an introducer does not cause any conversion; it is strictly a signal that does not change the string's value. An introducer is also legal before standard hex literal and numeric hex literal notation (x'literal' and 0xnnnn)>.

Examples:

SELECT _latin1 x'AABBCC';
SELECT _latin1 0xAABBCC;

MySQL determines a literal's character set and collation in the following manner:

  • If both _X and COLLATE Y were specified, then character set X and collation Y are used.

  • If _X is specified but COLLATE is not specified, then character set X and its default collation are used.

  • Otherwise, the character set and collation given by the character_set_connection and collation_connection system variables are used.

Examples:

  • A string with latin1 character set and latin1_german1_ci collation:

    SELECT _latin1'Mьller' COLLATE latin1_german1_ci;
  • A string with latin1 character set and its default collation (that is, latin1_swedish_ci):

    SELECT _latin1'Mьller';
  • A string with the connection default character set and collation:

    SELECT 'Mьller';

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

An introducer indicates the character set for the following string, but does not change now 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 processsing 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;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT HEX('а\n'), HEX(_sjis'а\n');
+------------+-----------------+
| HEX('а\n') | HEX(_sjis'а\n') |
+------------+-----------------+
| E00A       | E00A            | 
+------------+-----------------+
1 row in set (0.00 sec)

Here, ‘а’ (hex 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 (hex value 0A). This happens even for the second string. That is, the introducer of _sjis does not affect the parser's escape processing.

Example 2:

mysql> SET NAMES sjis;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT HEX('а\n'), HEX(_latin1'а\n');
+------------+-------------------+
| HEX('а\n') | HEX(_latin1'а\n') |
+------------+-------------------+
| E05C6E     | E05C6E            | 
+------------+-------------------+
1 row in set (0.04 sec)

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

10.3.6. National Character Set

Standard SQL defines NCHAR or NATIONAL CHAR as a way to indicate that a CHAR column should use some predefined character set. MySQL 5.0 uses utf8 as this predefined character set. For example, these data type declarations are equivalent:

CHAR(10) CHARACTER SET utf8
NATIONAL CHARACTER(10)
NCHAR(10)

As are these:

VARCHAR(10) CHARACTER SET utf8
NATIONAL VARCHAR(10)
NCHAR VARCHAR(10)
NATIONAL CHARACTER VARYING(10)
NATIONAL CHAR VARYING(10)

You can use N'literal' to create a string in the national character set. These two statements are equivalent:

SELECT N'some text';
SELECT _utf8'some text';

For information on upgrading character sets to MySQL 5.0 from versions prior to 4.1, see the MySQL 3.23, 4.0, 4.1 Reference Manual.

10.3.7. Examples of Character Set and Collation Assignment

The following examples show how MySQL determines default character set and collation values.

Example 1: Table and Column Definition

CREATE TABLE t1
(
    c1 CHAR(10) CHARACTER SET latin1 COLLATE latin1_german1_ci
) DEFAULT CHARACTER SET latin2 COLLATE latin2_bin;

Here we have a column with a latin1 character set and a latin1_german1_ci collation. The definition is explicit, so that's straightforward. Notice that there is no problem with storing a latin1 column in a latin2 table.

Example 2: Table and Column Definition

CREATE TABLE t1
(
    c1 CHAR(10) CHARACTER SET latin1
) DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

This time we have a column with a latin1 character set and a default collation. Although it might seem natural, the default collation is not taken from the table level. Instead, because the default collation for latin1 is always latin1_swedish_ci, column c1 has a collation of latin1_swedish_ci (not latin1_danish_ci).

Example 3: Table and Column Definition

CREATE TABLE t1
(
    c1 CHAR(10)
) DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

We have a column with a default character set and a default collation. In this circumstance, MySQL checks the table level to determine the column character set and collation. Consequently, the character set for column c1 is latin1 and its collation is latin1_danish_ci.

Example 4: Database, Table, and Column Definition

CREATE DATABASE d1
    DEFAULT CHARACTER SET latin2 COLLATE latin2_czech_ci;
USE d1;
CREATE TABLE t1
(
    c1 CHAR(10)
);

We create a column without specifying its character set and collation. We're also not specifying a character set and a collation at the table level. In this circumstance, MySQL checks the database level to determine the table settings, which thereafter become the column settings.) Consequently, the character set for column c1 is latin2 and its collation is latin2_czech_ci.

10.3.8. Compatibility with Other DBMSs

For MaxDB compatibility these two statements are the same:

CREATE TABLE t1 (f1 CHAR(N) UNICODE);
CREATE TABLE t1 (f1 CHAR(N) CHARACTER SET ucs2);

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