MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6
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 10.14.3, “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 10.13, “Adding a Character Set”.
Collations for non-Unicode multibyte character sets
For this type of collation, 8-bit (single-byte) and multibyte
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 multibyte 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 multibyte 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: 0 mysql>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 multibyte 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: 0 mysql>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 10.13, “Adding a Character Set”.
Collations for Unicode multibyte 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: 0 mysql>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 10.13, “Adding a Character Set”. For a UCA collation, see Section 10.14.4, “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.