MySQL原理 - 字符集与排序规则
任何计算机存储数据,都需要字符集,因为计算机存储的数据其实都是二进制编码,将一个个字符,映射到对应的二进制编码的这个映射就是字符编码(字符集)。这些字符如何排序呢?决定字符排序的规则就是排序规则。
查看内置字符集与比较规则
通过show charset;
命令,可以查看所有的字符集。 以下仅展示了我们常用的字符集:
+----------+---------------------------------+---------------------+--------+ | Charset | Description | Default collation | Maxlen | +----------+---------------------------------+---------------------+--------+ | latin1 | cp1252 West European | latin1_swedish_ci | 1 | | ascii | US ASCII | ascii_general_ci | 1 | | gb2312 | GB2312 Simplified Chinese | gb2312_chinese_ci | 2 | | cp1250 | Windows Central European | cp1250_general_ci | 1 | | gbk | GBK Simplified Chinese | gbk_chinese_ci | 2 | | utf8 | UTF-8 Unicode | utf8_general_ci | 3 | | utf8mb4 | UTF-8 Unicode | utf8mb4_general_ci | 4 | | utf16 | UTF-16 Unicode | utf16_general_ci | 4 | | utf32 | UTF-32 Unicode | utf32_general_ci | 4 | +----------+---------------------------------+---------------------+--------+
ascii
:共收录128个字符,包括空格、标点符号、数字、大小写字母和一些不可见字符。由于总共才128个字符,所以可以使用1个字节来进行编码latin1
:共收录256个字符,是在ASCII字符集的基础上又扩充了128个西欧常用字符(包括德法两国的字母),也可以使用1个字节来进行编码。gb2312
: 收录了汉字以及拉丁字母、希腊字母、日文平假名及片假名字母、俄语西里尔字母。其中收录汉字6763个,其他文字符号682个,兼容ASCII字符集。这是一个变长字符集,如果该字符在ascii
字符集中,则采用1字节编码,否则采用两字节。gbk
: GBK是在gb2312
基础上扩容后的标准。收录了所有的中文字符。同样的,这是一个变长字符集,如果该字符在ascii
字符集中,则采用1字节编码,否则采用两字节。utf8
和utf8mb4
: 收录地球上能想到的所有字符,而且还在不断扩充。这种字符集兼容ASCII字符集,采用变长编码方式,编码一个字符需要使用1~4个字节。MySQL
为了节省空间,其中的utf8
是标准 UTF8 阉割后的,只有1~3字节编码的字符集,基本包含了所有常用的字符。如果还要使用 enoji 表情,那么需要使用utf8mb4
,这个是完整的 UTF8 字符集。utf16
: 不同于utf8
,utf16
用两个字节或者四个字节编码字符,可以理解为utf8
的不节省空间的一种形式utf32
: 固定用四个字节编码字符,可以理解为utf8
的不节省空间的一种形式
通过查看information_schema.character_sets
表,也可以看到所有的字符集:
mysql> select * from information_schema.character_sets where character_set_name = "utf8"; +--------------------+----------------------+---------------+--------+ | CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION | MAXLEN | +--------------------+----------------------+---------------+--------+ | utf8 | utf8_general_ci | UTF-8 Unicode | 3 | +--------------------+----------------------+---------------+--------+ 1 row in set (0.06 sec)
通过show collation;
命令,可以查看所有的字符集,我们这里来查看utf8mb4
的排序规则:
mysql> show collation like ‘utf8mb4%‘; +------------------------+---------+-----+---------+----------+---------+ | Collation | Charset | Id | Default | Compiled | Sortlen | +------------------------+---------+-----+---------+----------+---------+ | utf8mb4_general_ci | utf8mb4 | 45 | Yes | Yes | 1 | | utf8mb4_bin | utf8mb4 | 46 | | Yes | 1 | | utf8mb4_unicode_ci | utf8mb4 | 224 | | Yes | 8 | | utf8mb4_icelandic_ci | utf8mb4 | 225 | | Yes | 8 | | utf8mb4_latvian_ci | utf8mb4 | 226 | | Yes | 8 | | utf8mb4_romanian_ci | utf8mb4 | 227 | | Yes | 8 | | utf8mb4_slovenian_ci | utf8mb4 | 228 | | Yes | 8 | | utf8mb4_polish_ci | utf8mb4 | 229 | | Yes | 8 | | utf8mb4_estonian_ci | utf8mb4 | 230 | | Yes | 8 | | utf8mb4_spanish_ci | utf8mb4 | 231 | | Yes | 8 | | utf8mb4_swedish_ci | utf8mb4 | 232 | | Yes | 8 | | utf8mb4_turkish_ci | utf8mb4 | 233 | | Yes | 8 | | utf8mb4_czech_ci | utf8mb4 | 234 | | Yes | 8 | | utf8mb4_danish_ci | utf8mb4 | 235 | | Yes | 8 | | utf8mb4_lithuanian_ci | utf8mb4 | 236 | | Yes | 8 | | utf8mb4_slovak_ci | utf8mb4 | 237 | | Yes | 8 | | utf8mb4_spanish2_ci | utf8mb4 | 238 | | Yes | 8 | | utf8mb4_roman_ci | utf8mb4 | 239 | | Yes | 8 | | utf8mb4_persian_ci | utf8mb4 | 240 | | Yes | 8 | | utf8mb4_esperanto_ci | utf8mb4 | 241 | | Yes | 8 | | utf8mb4_hungarian_ci | utf8mb4 | 242 | | Yes | 8 | | utf8mb4_sinhala_ci | utf8mb4 | 243 | | Yes | 8 | | utf8mb4_german2_ci | utf8mb4 | 244 | | Yes | 8 | | utf8mb4_croatian_ci | utf8mb4 | 245 | | Yes | 8 | | utf8mb4_unicode_520_ci | utf8mb4 | 246 | | Yes | 8 | | utf8mb4_vietnamese_ci | utf8mb4 | 247 | | Yes | 8 | +------------------------+---------+-----+---------+----------+---------+ 26 rows in set (0.13 sec)
同样的,通过查询information_schema.collations
也可以:
mysql> select * from information_schema.collations where character_set_name = "utf8mb4"; +------------------------+--------------------+-----+------------+-------------+---------+ | COLLATION_NAME | CHARACTER_SET_NAME | ID | IS_DEFAULT | IS_COMPILED | SORTLEN | +------------------------+--------------------+-----+------------+-------------+---------+ | utf8mb4_general_ci | utf8mb4 | 45 | Yes | Yes | 1 | | utf8mb4_bin | utf8mb4 | 46 | | Yes | 1 | | utf8mb4_unicode_ci | utf8mb4 | 224 | | Yes | 8 | | utf8mb4_icelandic_ci | utf8mb4 | 225 | | Yes | 8 | | utf8mb4_latvian_ci | utf8mb4 | 226 | | Yes | 8 | | utf8mb4_romanian_ci | utf8mb4 | 227 | | Yes | 8 | | utf8mb4_slovenian_ci | utf8mb4 | 228 | | Yes | 8 | | utf8mb4_polish_ci | utf8mb4 | 229 | | Yes | 8 | | utf8mb4_estonian_ci | utf8mb4 | 230 | | Yes | 8 | | utf8mb4_spanish_ci | utf8mb4 | 231 | | Yes | 8 | | utf8mb4_swedish_ci | utf8mb4 | 232 | | Yes | 8 | | utf8mb4_turkish_ci | utf8mb4 | 233 | | Yes | 8 | | utf8mb4_czech_ci | utf8mb4 | 234 | | Yes | 8 | | utf8mb4_danish_ci | utf8mb4 | 235 | | Yes | 8 | | utf8mb4_lithuanian_ci | utf8mb4 | 236 | | Yes | 8 | | utf8mb4_slovak_ci | utf8mb4 | 237 | | Yes | 8 | | utf8mb4_spanish2_ci | utf8mb4 | 238 | | Yes | 8 | | utf8mb4_roman_ci | utf8mb4 | 239 | | Yes | 8 | | utf8mb4_persian_ci | utf8mb4 | 240 | | Yes | 8 | | utf8mb4_esperanto_ci | utf8mb4 | 241 | | Yes | 8 | | utf8mb4_hungarian_ci | utf8mb4 | 242 | | Yes | 8 | | utf8mb4_sinhala_ci | utf8mb4 | 243 | | Yes | 8 | | utf8mb4_german2_ci | utf8mb4 | 244 | | Yes | 8 | | utf8mb4_croatian_ci | utf8mb4 | 245 | | Yes | 8 | | utf8mb4_unicode_520_ci | utf8mb4 | 246 | | Yes | 8 | | utf8mb4_vietnamese_ci | utf8mb4 | 247 | | Yes | 8 | +------------------------+--------------------+-----+------------+-------------+---------+ 26 rows in set (0.11 sec)
- 每个字符集都有一个默认的排序规则:IS_DEFAULT 为 Yes。
- 比较规则名称以与其关联的字符集的名称开头,可以用通过这个开头查询所有的字符集,也可以查询
information_schema.collations
精确指定字符集 - 字符集后面跟着的是语言编码,因为
utf8mb4
包含了所有字符,不同国家的文字语言排序肯定不一样。 - 最后末尾的
ci
代表case insensitive
,大小写不敏感,所有可能的后缀如下所示:- ai: accent insensitive 不区分重音
- as: accent sensitive 区分重音
- ci: case insensitive 不区分大小写
- cs: case sensitive 区分大小写
- bin: binary 以二进制方式比较
应用字符集与比较规则
字符集与比较规则配置有四个级别:
- MySQL实例级别
- 库级别
- 表级别
- 字段级别 指定的级别粒度越小,则以粒度越小的字符集还有比较规则优先。例如指定MySQL实例级别字符集是
utf8mb4
,指定某个表字符集是latin1
,那么这个表的所有字段如果不指定的话,编码就是latin1
由于字符集和比较规则是互相有联系的,如果我们只修改了字符集,比较规则也会跟着变化,如果只修改了比较规则,字符集也会跟着变化,具体规则如下:
- 只修改字符集,则比较规则将变为修改后的字符集默认的比较规则。
- 只修改比较规则,则字符集将变为修改后的比较规则对应的字符集。
实例级别
通过两个系统变量来指定实例级别的字符集与排序规则。
配置文件:
[server] character_set_server=utf8mb4 collation_server=utf8mb4_general_ci
启动之后,可以查看并修改这两个变量。
mysql> show variables like ‘character_set_server‘; +----------------------+---------+ | Variable_name | Value | +----------------------+---------+ | character_set_server | utf8mb4 | +----------------------+---------+ 1 row in set (0.06 sec) mysql> show variables like ‘collation_server‘; +------------------+--------------------+ | Variable_name | Value | +------------------+--------------------+ | collation_server | utf8mb4_general_ci | +------------------+--------------------+ 1 row in set (0.05 sec) mysql> set character_set_server = ‘utf8mb4‘; Query OK, 0 rows affected (0.00 sec) mysql> set collation_server = ‘utf8mb4_general_ci‘; Query OK, 0 rows affected (0.00 sec)
库级别
创建数据库的时候,可以指定字符集还有排序规则。
mysql> create database test_db character set utf8mb4 collate utf8mb4_general_ci; Query OK, 1 row affected (0.01 sec)
不指定的话,就用实例级别的字符集还有排序规则。
查看当前数据库的字符集还有排序规则则是通过use
命令指定数据库之后,查看character_set_database
变量以及collation_database
来实现:
mysql> show variables like ‘character_set_database‘; +------------------------+---------+ | Variable_name | Value | +------------------------+---------+ | character_set_database | utf8mb4 | +------------------------+---------+ 1 row in set (0.07 sec) mysql> show variables like ‘collation_database‘; +--------------------+--------------------+ | Variable_name | Value | +--------------------+--------------------+ | collation_database | utf8mb4_general_ci | +--------------------+--------------------+ 1 row in set (0.09 sec)
就算设置这两个变量,也是无效的:
mysql> set character_set_database = ‘utf8‘; Query OK, 0 rows affected (0.00 sec) mysql> show variables like ‘character_set_database‘; +------------------------+---------+ | Variable_name | Value | +------------------------+---------+ | character_set_database | utf8mb4 | +------------------------+---------+ 1 row in set (0.09 sec)
修改数据库的字符集还有排序规则的方式:
mysql> alter database test_db character set = ‘utf8‘; Query OK, 1 row affected (0.01 sec) mysql> show variables like ‘character_set_database‘; +------------------------+-------+ | Variable_name | Value | +------------------------+-------+ | character_set_database | utf8 | +------------------------+-------+ 1 row in set (0.08 sec)
这个更新只会对新建的表如果没指定字符集和排序规则的生效,并不会更新老表的字符集还有排序规则。
表级别
可以在创建时指定字符集合排序规则,不指定的话,用数据库的字符集还有排序规则,也可以修改字符集和排序规则。
mysql> create table test (name varchar(32)) character set utf8mb4 collate utf8mb4_bin; Query OK, 0 rows affected (0.04 sec) mysql> show create table test; +-------+---------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+---------------------------------------------------------------------------------------------------------------------------------------+ | test | CREATE TABLE `test` ( `name` varchar(32) COLLATE utf8mb4_bin DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin | +-------+---------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.09 sec) mysql> alter table test character set = ‘utf8‘; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> show create table test; +-------+--------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+--------------------------------------------------------------------------------------------------------------------------------------+ | test | CREATE TABLE `test` ( `name` varchar(32) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +-------+--------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.06 sec)
可以看出,仅仅是表的字符集还有排序规则变了,对于已有字段,并没有改变编码和排序规则。
列级别
可以在创建表的时候,指定不同的列有不同的字符集和排序规则,也可以修改列的字符集和排序规则:
mysql> create table test (name varchar(32) character set utf8 collate utf8_bin) character set utf8mb4 collate utf8mb4_bin; Query OK, 0 rows affected (0.03 sec) mysql> show create table test; +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------+ | test | CREATE TABLE `test` ( `name` varchar(32) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin | +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.09 sec) mysql> alter table test modify column name varchar(32) COLLATE latin1_bin; Query OK, 0 rows affected (0.09 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> show create table test; +-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+ | test | CREATE TABLE `test` ( `name` varchar(32) CHARACTER SET latin1 COLLATE latin1_bin DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin | +-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.09 sec)
MySQL客户端字符编码问题
有时候,我们会遇到字符编码不一致导致的程序问题。例如我们的 Java 程序,使用 jdbc 链接。读取的数据,打印出来是乱码。或者是,MySQL 无法识别我们客户端发来的命令。这涉及到字符编码问题。我们需要保持 Java 程序的字符编码与 JDBC 链接指定的字符编码一致,这样才不会有乱码的问题。
**指定 Java 程序编码:**通过启动参数:-Dfile.encoding=UTF-8
设置默认的字符编码(java.nio.charset.Charset.defaultCharset();
)是utf-8
(对应 MySQL 的utf8
还有utf8mb4
)。
指定 JDBC 链接编码:
jdbc:mysql://127.0.0.1:3306/test?characterEncoding=utf8
mysql客户端命令行指定字符集
mysql -h 127.0.0.1 -P 3306 -u root --default-character-set=utf8mb4 -p
之后查看有关编码的环境变量,都是和设置的这个字符集一样。
mysql> SHOW VARIABLES LIKE ‘character_set_client‘; +----------------------+---------+ | Variable_name | Value | +----------------------+---------+ | character_set_client | utf8mb4 | +----------------------+---------+ 1 row in set, 1 warning (0.00 sec) mysql> SHOW VARIABLES LIKE ‘character_set_connection‘; +--------------------------+---------+ | Variable_name | Value | +--------------------------+---------+ | character_set_connection | utf8mb4 | +--------------------------+---------+ 1 row in set, 1 warning (0.00 sec) mysql> SHOW VARIABLES LIKE ‘character_set_results‘; +-----------------------+---------+ | Variable_name | Value | +-----------------------+---------+ | character_set_results | utf8mb4 | +-----------------------+---------+ 1 row in set, 1 warning (0.00 sec)
其中:
character_set_client
: 服务器解码请求时使用的字符集character_set_connection
:服务器处理请求时将字符集转换成这个字符集处理。操作具体列时,在转换为具体列的编码。character_set_results
:服务器向客户端返回数据时使用的字符集
MySQL 设计这三个编码的时候,出于以下考虑:
- 一个 MySQL,可能有多种不同语言和操作系统或者国家的客户端,所以通过设置
character_set_client
还有character_set_results
进行兼容。 - 由于操作具体列数据的时候需要编码转换,如果
character_set_connection
和字段一致的话,就不用转换了,所以设置character_set_connection
可以让 MySQL 用一种编码理解命令统一处理,同时设置character_set_connection
为最常用的可以减少转换。
一般情况下,保持这三个一致就好。我们就设置好连接使用的字符集就行了。