10.4.1. Collation Implementation Types

MySQL implements several types of collations:

Simple collations for 8-bit character sets

This kind of collation is implemented using an array of 256 weights that defines a one-to-one mapping from character codes to weights. latin1_swedish_ci is an example. It is a case-insensitive collation, so the uppercase and lowercase versions of a character have the same weights and they compare as equal.

mysql> SET NAMES 'latin1' COLLATE
        'latin1_swedish_ci';Query OK, 0 rows affected (0.01 sec)mysql> SELECT HEX(WEIGHT_STRING('a')), HEX(WEIGHT_STRING('A'));+-------------------------+-------------------------+| HEX(WEIGHT_STRING('a')) | HEX(WEIGHT_STRING('A')) |+-------------------------+-------------------------+| 41                      | 41                      |+-------------------------+-------------------------+1 row in set (0.01 sec)mysql> SELECT 'a' = 'A';+-----------+| 'a' = 'A' |+-----------+|         1 |+-----------+1 row in set (0.12 sec)

For implementation instructions, see Section 10.4.3, "Adding a Simple Collation to an 8-Bit Character Set".

Complex collations for 8-bit character sets

This kind of collation is implemented using functions in a C source file that define how to order characters, as described in Section 10.3, "Adding a Character Set".

Collations for non-Unicode multi-byte character sets

For this type of collation, 8-bit (single-byte) and multi-byte characters are handled differently. For 8-bit characters, character codes map to weights in case-insensitive fashion. (For example, the single-byte characters 'a' and 'A' both have a weight of 0x41.) For multi-byte characters, there are two types of relationship between character codes and weights:

Weights equal character codes. sjis_japanese_ci is an example of this kind of collation. The multi-byte character 'ぢ' has a character code of 0x82C0, and the weight is also 0x82C0.

mysql> CREATE TABLE t1    -> (c1 VARCHAR(2) CHARACTER SET sjis COLLATE
                    sjis_japanese_ci);Query OK, 0 rows affected (0.01 sec)mysql> INSERT INTO t1 VALUES ('a'),('A'),(0x82C0);Query OK, 3 rows affected (0.00 sec)Records: 3  Duplicates: 0  Warnings: 0mysql> SELECT c1, HEX(c1), HEX(WEIGHT_STRING(c1)) FROM t1;+------+---------+------------------------+| c1   | HEX(c1) | HEX(WEIGHT_STRING(c1)) |+------+---------+------------------------+| a    | 61      | 41                     || A    | 41      | 41                     || ぢ    | 82C0    | 82C0                   |+------+---------+------------------------+3 rows in set (0.00 sec)

Character codes map one-to-one to weights, but a code is not necessarily equal to the weight. gbk_chinese_ci is an example of this kind of collation. The multi-byte character '膰' has a character code of 0x81B0 but a weight of 0xC286.

mysql> CREATE TABLE t1    -> (c1 VARCHAR(2) CHARACTER SET gbk COLLATE
                    gbk_chinese_ci);Query OK, 0 rows affected (0.33 sec)mysql> INSERT INTO t1 VALUES ('a'),('A'),(0x81B0);Query OK, 3 rows affected (0.00 sec)Records: 3  Duplicates: 0  Warnings: 0mysql> SELECT c1, HEX(c1), HEX(WEIGHT_STRING(c1)) FROM t1;+------+---------+------------------------+| c1   | HEX(c1) | HEX(WEIGHT_STRING(c1)) |+------+---------+------------------------+| a    | 61      | 41                     || A    | 41      | 41                     || 膰    | 81B0    | C286                   |+------+---------+------------------------+3 rows in set (0.00 sec)

For implementation instructions, see Section 10.3, "Adding a Character Set".

Collations for Unicode multi-byte character sets

Some of these collations are based on the Unicode Collation Algorithm (UCA), others are not.

Non-UCA collations have a one-to-one mapping from character code to weight. In MySQL, such collations are case insensitive and accent insensitive. utf8_general_ci is an example: 'a', 'A', 'À', and 'á' each have different character codes but all have a weight of 0x0041 and compare as equal.

mysql> SET NAMES 'utf8' COLLATE
        'utf8_general_ci';Query OK, 0 rows affected (0.00 sec)mysql> CREATE
        TABLE t1    -> (c1 CHAR(1) CHARACTER SET UTF8 COLLATE
        utf8_general_ci);Query OK, 0 rows affected (0.01 sec)mysql> INSERT
        INTO t1 VALUES ('a'),('A'),('À'),('á');Query OK, 4 rows affected (0.00 sec)Records: 4  Duplicates: 0  Warnings: 0mysql> SELECT c1, HEX(c1), HEX(WEIGHT_STRING(c1)) FROM t1;+------+---------+------------------------+| c1   | HEX(c1) | HEX(WEIGHT_STRING(c1)) |+------+---------+------------------------+| a    | 61      | 0041                   || A    | 41      | 0041                   || À    | C380    | 0041                   || á    | C3A1    | 0041                   |+------+---------+------------------------+4 rows in set (0.00 sec)

UCA-based collations in MySQL have these properties:

If a character has weights, each weight uses 2 bytes (16 bits).
A character may have zero weights (or an empty weight). In this case, the character is ignorable. Example: "U+0000 NULL" does not have a weight and is ignorable.

A character may have one weight. Example: 'a' has a weight of 0x0E33.

mysql> SET NAMES 'utf8' COLLATE
                    'utf8_unicode_ci';Query OK, 0 rows affected (0.05 sec)mysql> SELECT HEX('a'), HEX(WEIGHT_STRING('a'));+----------+-------------------------+| HEX('a') | HEX(WEIGHT_STRING('a')) |+----------+-------------------------+| 61       | 0E33                    |+----------+-------------------------+1 row in set (0.02 sec)

A character may have many weights. This is an expansion. Example: The German letter 'ß' (SZ ligature, or SHARP S) has a weight of 0x0FEA0FEA.

mysql> SET NAMES 'utf8' COLLATE
                    'utf8_unicode_ci';Query OK, 0 rows affected (0.11 sec)mysql> SELECT HEX('ß'), HEX(WEIGHT_STRING('ß'));+-----------+--------------------------+| HEX('ß')  | HEX(WEIGHT_STRING('ß'))  |+-----------+--------------------------+| C39F      | 0FEA0FEA                 |+-----------+--------------------------+1 row in set (0.00 sec)

Many characters may have one weight. This is a contraction. Example: 'ch' is a single letter in Czech and has a weight of 0x0EE2.

mysql> SET NAMES 'utf8' COLLATE
                    'utf8_czech_ci';Query OK, 0 rows affected (0.09 sec)mysql> SELECT HEX('ch'), HEX(WEIGHT_STRING('ch'));+-----------+--------------------------+| HEX('ch') | HEX(WEIGHT_STRING('ch')) |+-----------+--------------------------+| 6368      | 0EE2                     |+-----------+--------------------------+1 row in set (0.00 sec)

A many-characters-to-many-weights mapping is also possible (this is contraction with expansion), but is not supported by MySQL.

For implementation instructions, for a non-UCA collation, see Section 10.3, "Adding a Character Set". For a UCA collation, see Section 10.4.4, "Adding a UCA Collation to a Unicode Character Set".

Miscellaneous collations

There are also a few collations that do not fall into any of the previous categories.