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10.13.1 Collation Implementation Types

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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.

  1. mysql> SET NAMES 'latin1' COLLATE 'latin1_swedish_ci';
  2. Query OK, 0 rows affected (0.01 sec)
  3.  
  4. mysql> SELECT HEX(WEIGHT_STRING('a')), HEX(WEIGHT_STRING('A'));
  5. +-------------------------+-------------------------+
  6. | HEX(WEIGHT_STRING('a')) | HEX(WEIGHT_STRING('A')) |
  7. +-------------------------+-------------------------+
  8. | 41                      | 41                      |
  9. +-------------------------+-------------------------+
  10. 1 row in set (0.01 sec)
  11.  
  12. mysql> SELECT 'a' = 'A';
  13. +-----------+
  14. | 'a' = 'A' |
  15. +-----------+
  16. |         1 |
  17. +-----------+
  18. 1 row in set (0.12 sec)

For implementation instructions, see Section 10.13.3, “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 10.12, “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.

    1. mysql> CREATE TABLE t1
    2.        (c1 VARCHAR(2) CHARACTER SET sjis COLLATE sjis_japanese_ci);
    3. Query OK, 0 rows affected (0.01 sec)
    4.  
    5. mysql> INSERT INTO t1 VALUES ('a'),('A'),(0x82C0);
    6. Query OK, 3 rows affected (0.00 sec)
    7. Records: 3  Duplicates: 0  Warnings: 0
    8.  
    9. mysql> SELECT c1, HEX(c1), HEX(WEIGHT_STRING(c1)) FROM t1;
    10. +------+---------+------------------------+
    11. | c1   | HEX(c1) | HEX(WEIGHT_STRING(c1)) |
    12. +------+---------+------------------------+
    13. | a    | 61      | 41                     |
    14. | A    | 41      | 41                     |
    15. | ぢ    | 82C0    | 82C0                   |
    16. +------+---------+------------------------+
    17. 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.

    1. mysql> CREATE TABLE t1
    2.        (c1 VARCHAR(2) CHARACTER SET gbk COLLATE gbk_chinese_ci);
    3. Query OK, 0 rows affected (0.33 sec)
    4.  
    5. mysql> INSERT INTO t1 VALUES ('a'),('A'),(0x81B0);
    6. Query OK, 3 rows affected (0.00 sec)
    7. Records: 3  Duplicates: 0  Warnings: 0
    8.  
    9. mysql> SELECT c1, HEX(c1), HEX(WEIGHT_STRING(c1)) FROM t1;
    10. +------+---------+------------------------+
    11. | c1   | HEX(c1) | HEX(WEIGHT_STRING(c1)) |
    12. +------+---------+------------------------+
    13. | a    | 61      | 41                     |
    14. | A    | 41      | 41                     |
    15. | 膰    | 81B0    | C286                   |
    16. +------+---------+------------------------+
    17. 3 rows in set (0.00 sec)

For implementation instructions, see Section 10.12, “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.

  1. mysql> SET NAMES 'utf8' COLLATE 'utf8_general_ci';
  2. Query OK, 0 rows affected (0.00 sec)
  3.  
  4. mysql> CREATE TABLE t1
  5.        (c1 CHAR(1) CHARACTER SET UTF8 COLLATE utf8_general_ci);
  6. Query OK, 0 rows affected (0.01 sec)
  7.  
  8. mysql> INSERT INTO t1 VALUES ('a'),('A'),('À'),('á');
  9. Query OK, 4 rows affected (0.00 sec)
  10. Records: 4  Duplicates: 0  Warnings: 0
  11.  
  12. mysql> SELECT c1, HEX(c1), HEX(WEIGHT_STRING(c1)) FROM t1;
  13. +------+---------+------------------------+
  14. | c1   | HEX(c1) | HEX(WEIGHT_STRING(c1)) |
  15. +------+---------+------------------------+
  16. | a    | 61      | 0041                   |
  17. | A    | 41      | 0041                   |
  18. | À    | C380    | 0041                   |
  19. | á    | C3A1    | 0041                   |
  20. +------+---------+------------------------+
  21. 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.

    1. mysql> SET NAMES 'utf8' COLLATE 'utf8_unicode_ci';
    2. Query OK, 0 rows affected (0.05 sec)
    3.  
    4. mysql> SELECT HEX('a'), HEX(WEIGHT_STRING('a'));
    5. +----------+-------------------------+
    6. | HEX('a') | HEX(WEIGHT_STRING('a')) |
    7. +----------+-------------------------+
    8. | 61       | 0E33                    |
    9. +----------+-------------------------+
    10. 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.

    1. mysql> SET NAMES 'utf8' COLLATE 'utf8_unicode_ci';
    2. Query OK, 0 rows affected (0.11 sec)
    3.  
    4. mysql> SELECT HEX('ß'), HEX(WEIGHT_STRING('ß'));
    5. +-----------+--------------------------+
    6. | HEX('ß')  | HEX(WEIGHT_STRING('ß'))  |
    7. +-----------+--------------------------+
    8. | C39F      | 0FEA0FEA                 |
    9. +-----------+--------------------------+
    10. 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.

    1. mysql> SET NAMES 'utf8' COLLATE 'utf8_czech_ci';
    2. Query OK, 0 rows affected (0.09 sec)
    3.  
    4. mysql> SELECT HEX('ch'), HEX(WEIGHT_STRING('ch'));
    5. +-----------+--------------------------+
    6. | HEX('ch') | HEX(WEIGHT_STRING('ch')) |
    7. +-----------+--------------------------+
    8. | 6368      | 0EE2                     |
    9. +-----------+--------------------------+
    10. 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 10.12, “Adding a Character Set”. For a UCA collation, see Section 10.13.4, “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.


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Dokument erstellt 26/06/2006, zuletzt geändert 26/10/2018
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