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）：
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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)
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查看表默认排序规则集
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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)
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查看列排序规则集
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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)
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从utf8升级为utf8mb4是不支持online ddl的，如下：
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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.
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从utf8.utf8_general_ci变更为utf8.utf8_unicode_ci是不支持online ddl的，如下：
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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.
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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。
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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)
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所以真正改字符集的时候别忘了加上CONVERT TO，如下：
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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)
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要仅仅改变一个表的默认字符集，应使用此语句：
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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)
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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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