字符集
字符集是字符的编码规则，校对规则是对字符集中的字符比较大小。由于字符集和校对规则之间相互关联，因此如果只修改字符集，校对规则也会跟着变化，而如果只修改校对规则，字符集则也会跟着变化。
StoneDB支持的字符集
> 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 |
+----------+---------------------------------+---------------------+--------+
字符集的变量
> show variables like '%char%';
+--------------------------+----------------------------------+
| Variable_name | Value |
+--------------------------+----------------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | utf8mb4 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | utf8mb4 |
| character_set_system | utf8 |
| character_sets_dir | /stonedb/install/share/charsets/ |
+--------------------------+----------------------------------+
8 rows in set (0.01 sec)
- 客户端向服务端发送请求使用的字符集：取决于操作系统的字符集，LC_ALL、LC_CTYPE、LANG 决定了操作系统使用的字符集，优先级依次从高到底。
- 服务端接收客户端请求使用的字符集：取决于变量 character_set_client
- 服务端在运行过程中转换使用的字符集：取决于变量 character_set_connection
- 服务端向客户端返回请求使用的字符集：取决于变量 character_set_results
- 客户端接收服务端响应使用的字符集：取决于操作系统的字符集
假设服务端使用的字符集为 utf8，客户端启动时使用了 --default-character-set=gbk 选项，那么character_set_client、character_set_connection、character_set_results 都会被设置为 gbk。假如某张表中存储了中文，那么服务端和客户端这些中文的字符编码是不同的，在Unix操 作系统中服务端会根据character_set_connection 进行转换，很显然客户端收到的结果是乱码。
StoneDB 默认的字符集取决于变量 character_set_server，如果创建的数据库没有指定字符集，默认的字符集就是变量 character_set_server 的值。如果创建的表没有指定字符集，继承的是数据库的字符集。StoneDB 的表一旦被创建后，字符集就不能被修改和转换。