在《MySQL字符集和校对规则详解》一节里，已经把 MySQL 的字符集和校对规则讲清楚了。

这一节继续往下走，重点看一件更实用的事：怎么把字符集和校对规则查出来。

查看 MySQL 支持的字符集

在 MySQL 里，查看可用字符集，最直接的方式就是执行下面这条命令，执行过程如下：

mysql> SHOW CHARACTER set;
+----------+---------------------------------+---------------------+--------+
| Charset  | Description                     | Default collation   | Maxlen |
+----------+---------------------------------+---------------------+--------+
| big5     | Big5 Traditional Chinese        | big5_chinese_ci     |      2 |
| dec8     | DEC West European               | dec8_swedish_ci     |      1 |
| cp850    | DOS West European               | cp850_general_ci    |      1 |
| hp8      | HP West European                | hp8_english_ci      |      1 |
| koi8r    | KOI8-R Relcom Russian           | koi8r_general_ci    |      1 |
| latin1   | cp1252 West European            | latin1_swedish_ci   |      1 |
| latin2   | ISO 8859-2 Central European     | latin2_general_ci   |      1 |
| swe7     | 7bit Swedish                    | swe7_swedish_ci     |      1 |
| ascii    | US ASCII                        | ascii_general_ci    |      1 |
| ujis     | EUC-JP Japanese                 | ujis_japanese_ci    |      3 |
| sjis     | Shift-JIS Japanese              | sjis_japanese_ci    |      2 |
| hebrew   | ISO 8859-8 Hebrew               | hebrew_general_ci   |      1 |
| tis620   | TIS620 Thai                     | tis620_thai_ci      |      1 |
| euckr    | EUC-KR Korean                   | euckr_korean_ci     |      2 |
| koi8u    | KOI8-U Ukrainian                | koi8u_general_ci    |      1 |
| gb2312   | GB2312 Simplified Chinese       | gb2312_chinese_ci   |      2 |
| greek    | ISO 8859-7 Greek                | greek_general_ci    |      1 |
| cp1250   | Windows Central European        | cp1250_general_ci   |      1 |
| gbk      | GBK Simplified Chinese          | gbk_chinese_ci      |      2 |
| latin5   | ISO 8859-9 Turkish              | latin5_turkish_ci   |      1 |
| armscii8 | ARMSCII-8 Armenian              | armscii8_general_ci |      1 |
| utf8     | UTF-8 Unicode                   | utf8_general_ci     |      3 |
| ucs2     | UCS-2 Unicode                   | ucs2_general_ci     |      2 |
| cp866    | DOS Russian                     | cp866_general_ci    |      1 |
| keybcs2  | DOS Kamenicky Czech-Slovak      | keybcs2_general_ci  |      1 |
| macce    | Mac Central European            | macce_general_ci    |      1 |
| macroman | Mac West European               | macroman_general_ci |      1 |
| cp852    | DOS Central European            | cp852_general_ci    |      1 |
| latin7   | ISO 8859-13 Baltic              | latin7_general_ci   |      1 |
| utf8mb4  | UTF-8 Unicode                   | utf8mb4_general_ci  |      4 |
| cp1251   | Windows Cyrillic                | cp1251_general_ci   |      1 |
| utf16    | UTF-16 Unicode                  | utf16_general_ci    |      4 |
| utf16le  | UTF-16LE Unicode                | utf16le_general_ci  |      4 |
| cp1256   | Windows Arabic                  | cp1256_general_ci   |      1 |
| cp1257   | Windows Baltic                  | cp1257_general_ci   |      1 |
| utf32    | UTF-32 Unicode                  | utf32_general_ci    |      4 |
| binary   | Binary pseudo charset           | binary              |      1 |
| geostd8  | GEOSTD8 Georgian                | geostd8_general_ci  |      1 |
| cp932    | SJIS for Windows Japanese       | cp932_japanese_ci   |      2 |
| eucjpms  | UJIS for Windows Japanese       | eucjpms_japanese_ci |      3 |
| gb18030  | China National Standard GB18030 | gb18030_chinese_ci  |      4 |
+----------+---------------------------------+---------------------+--------+
41 rows in set (0.02 sec)

其中：

• 第一列（Charset）表示字符集名称；
• 第二列（Description）表示字符集的说明信息；
• 第三列（Default collation）表示该字符集对应的默认校对规则；
• 第四列（Maxlen）表示这个字符集中单个字符可能占用的最大字节数。

常见字符集

常见的字符集主要有下面这些：

• latin1 支持西欧字符、希腊字符等。
• gbk 支持中文简体字符。
• big5 支持中文繁体字符。
• utf8 几乎支持所有国家的字符。

通过系统表查看字符集

除了用命令查看，还可以直接查询 information_schema.character_set 表中的记录，来确认 MySQL 支持哪些字符集。

SQL 语句和执行过程如下：

mysql> SELECT * FROM information_schema.character_sets;
+--------------------+----------------------+---------------------------------+--------+
| CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION                     | MAXLEN |
+--------------------+----------------------+---------------------------------+--------+
| big5               | big5_chinese_ci      | Big5 Traditional Chinese        |      2 |
| dec8               | dec8_swedish_ci      | DEC West European               |      1 |
| cp850              | cp850_general_ci     | DOS West European               |      1 |
| hp8                | hp8_english_ci       | HP West European                |      1 |
......

查看某个字符集对应的校对规则

如果要看某个相关字符集对应的校对规则，可以使用 SHOW COLLATION LIKE '***'; 命令。

mysql> SHOW COLLATION LIKE 'gbk%';
+----------------+---------+----+---------+----------+---------+
| Collation      | Charset | Id | Default | Compiled | Sortlen |
+----------------+---------+----+---------+----------+---------+
| gbk_chinese_ci | gbk     | 28 | Yes     | Yes      |       1 |
| gbk_bin        | gbk     | 87 |         | Yes      |       1 |
+----------------+---------+----+---------+----------+---------+
2 rows in set (0.00 sec)

上面的结果展示的是 GBK 字符集对应的校对规则。

• gbk_chinese_ci 是默认校对规则，特点是对大小写不敏感；
• gbk_bin 则是按照二进制编码值进行比较，因此对大小写敏感。

通过系统表查看校对规则

同样，也可以通过查询 information_schema.COLLATIONS 表中的记录，来查看 MySQL 中可用的校对规则。

SQL 语句和执行过程如下：

mysql> SELECT * FROM information_schema.COLLATIONS;
+--------------------------+--------------------+-----+------------+-------------+---------+
| COLLATION_NAME           | CHARACTER_SET_NAME | ID  | IS_DEFAULT | IS_COMPILED | SORTLEN |
+--------------------------+--------------------+-----+------------+-------------+---------+
| big5_chinese_ci          | big5               |   1 | Yes        | Yes         |       1 |
| big5_bin                 | big5               |  84 |            | Yes         |       1 |
| dec8_swedish_ci          | dec8               |   3 | Yes        | Yes         |       1 |
| dec8_bin                 | dec8               |  69 |            | Yes         |       1 |
| cp850_general_ci         | cp850              |   4 | Yes        | Yes         |       1 |
| cp850_bin                | cp850              |  80 |            | Yes         |       1 |
......

例 1

分别指定“A”和“a”按照 gbk_chinese_ci 和 gbk_bin 校对规则进行比较。SQL 语句和运行结果如下：

mysql> SELECT CASE WHEN 'A' COLLATE gbk_chinese_ci = 'a' COLLATE gbk_chinese_ci then 1
    -> else 0 end;
+-------------------------------------------------------------------------------------+
| CASE WHEN 'A' COLLATE gbk_chinese_ci = 'a' COLLATE gbk_chinese_ci then 1
else 0 end |
+-------------------------------------------------------------------------------------+
|                                                                                   1 |
+-------------------------------------------------------------------------------------+
1 row in set (0.02 sec)

mysql> SELECT CASE WHEN 'A' COLLATE gbk_bin = 'a' COLLATE gbk_bin then 1
    -> else 0 end;
+-----------------------------------------------------------------------+
| CASE WHEN 'A' COLLATE gbk_bin = 'a' COLLATE gbk_bin then 1
else 0 end |
+-----------------------------------------------------------------------+
|                                                                     0 |
+-----------------------------------------------------------------------+
1 row in set (0.00 sec)

原因很直接：

• gbk_chinese_ci 校对规则会忽略大小写，所以会把 “A” 和 “a” 视为相同；
• gbk_bin 校对规则不会忽略大小写，因此会把这两个字符判定为不同。

在实际应用中，最好提前确认清楚应用到底需要按什么方式排序、是否需要区分大小写，再去选择对应的校对规则。