MySQL表排序规则不同错误问题分析

mysql多表join时报错如下：[err]1267 – illegal mix of collations(utf8_general_ci,implicit) and (utf8_unicode_ci,implicit) for operation ‘=

就是说两个表的排序规则（collation）不同，无法完成比较。collation是用在排序，大小比较上，一个字符集有一个或多种collation，并且以_ci（大小写不敏感）、_cs（大小写敏感）或_bin（二进制）结束。在做比较时，应该确保两个表的字符排序相同。一般建表的时候不指定，可以走默认的，全是默认的就没什么问题了。

下面来模拟一下各种场景，表结构如下（utf8默认排序规则为utf8_general_ci）：

mysql> show create table test.cs\g
*************************** 1. row ***************************
    table: cs
create table: create table `cs` (
 `id` int(11) default null,
 `name` varchar(10) default null
) engine=innodb default charset=utf8
1 row in set (0.01 sec)

查看表默认排序规则集

mysql> select table_schema,table_name,table_collation from information_schema.tables where table_name='cs';
+--------------+------------+-----------------+
| table_schema | table_name | table_collation |
+--------------+------------+-----------------+
| test     | cs     | utf8_general_ci |
+--------------+------------+-----------------+
1 row in set (0.00 sec)

查看列排序规则集

mysql> select table_schema,table_name,column_name,collation_name from information_schema.columns where table_name='cs';
+--------------+------------+-------------+-----------------+
| table_schema | table_name | column_name | collation_name |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | null      |
| test     | cs     | name    | utf8_general_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

从utf8升级为utf8mb4是不支持online ddl的，如下：

mysql> alter table cs convert to character set utf8mb4,algorithm=inplace,lock=none;
error 1846 (0a000): algorithm=inplace is not supported. reason: cannot change column type inplace. try algorithm=copy.

从utf8.utf8_general_ci变更为utf8.utf8_unicode_ci是不支持online ddl的，如下：

mysql> alter table cs convert to character set utf8 collate utf8_unicode_ci,algorithm=inplace,lock=none;
error 1846 (0a000): algorithm=inplace is not supported. reason: cannot change column type inplace. try algorithm=copy.

如果使用下面这种方式修改字符集，你会发现，只更改了表级的，没有更改列级的。

mysql> alter table cs character set utf8 collate utf8_unicode_ci;      
query ok, 0 rows affected (0.01 sec)
records: 0 duplicates: 0 warnings: 0
 
mysql> select table_schema,table_name,table_collation from information_schema.tables where table_name='cs';      
+--------------+------------+-----------------+
| table_schema | table_name | table_collation |
+--------------+------------+-----------------+
| test     | cs     | utf8_unicode_ci |
+--------------+------------+-----------------+
1 row in set (0.00 sec)
 
mysql> select table_schema,table_name,column_name,collation_name from information_schema.columns where table_name='cs';
+--------------+------------+-------------+-----------------+
| table_schema | table_name | column_name | collation_name |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | null      |
| test     | cs     | name    | utf8_general_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

所以真正改字符集的时候别忘了加上convert to，如下：

mysql> alter table cs convert to character set utf8 collate utf8_unicode_ci;
query ok, 5 rows affected (0.06 sec)
records: 5 duplicates: 0 warnings: 0
 
mysql> select table_schema,table_name,column_name,collation_name from information_schema.columns where table_name='cs';
+--------------+------------+-------------+-----------------+
| table_schema | table_name | column_name | collation_name |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | null      |
| test     | cs     | name    | utf8_unicode_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

要仅仅改变一个表的默认字符集，应使用此语句：

mysql> alter table cs default character set utf8 collate utf8_general_ci,algorithm=inplace,lock=none;         
query ok, 0 rows affected (0.00 sec)
records: 0 duplicates: 0 warnings: 0
 
mysql> select table_schema,table_name,table_collation from information_schema.tables where table_name='cs';      
+--------------+------------+-----------------+
| table_schema | table_name | table_collation |
+--------------+------------+-----------------+
| test     | cs     | utf8_general_ci |
+--------------+------------+-----------------+
1 row in set (0.00 sec)
 
mysql> select table_schema,table_name,column_name,collation_name from information_schema.columns where table_name='cs';  
+--------------+------------+-------------+-----------------+
| table_schema | table_name | column_name | collation_name |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | null      |
| test     | cs     | name    | utf8_unicode_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

可以发现列字符集没有改变，并且只有新的列才会默认继承表的字符集（utf8.utf8_general_ci）。

总结

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