MySQL多表join時報錯如下：[Err]1267 – Illegal mix of collations(utf8_general_ci,IMPLICIT) and (utf8_unicode_ci,IMPLICIT) for operation ‘=

就是說兩個表的排序規則（COLLATION）不同，無法完成比較。COLLATION是用在排序，大小比較上，一個字符集有一個或多種COLLATION，並且以_ci（大小寫不敏感）、_cs（大小寫敏感）或_bin（二進位制）結束。在做比較時，應該確保兩個表的字元排序相同。一般建表的時候不指定，可以走預設的，全是預設的就沒什麼問題了。

下面來模擬一下各種場景，表結構如下（utf8預設排序規則為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)

查看錶預設排序規則集

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)

檢視列排序規則集

mysql> select TABLE_SCHEMA,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)

從utf8升級為utf8mb4是不支援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.

從utf8.utf8_general_ci變更為utf8.utf8_unicode_ci是不支援online ddl的，如下：

mysql> ALTER TABLE cs CONVERT TO CHARACTER SET utf8 collate utf8_unicode_ci,LOCK=NONE;
ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

如果使用下面這種方式修改字符集，你會發現，只更改了表級的，沒有更改列級的。

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_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,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時候別忘了加上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,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)

要僅僅改變一個表的預設字符集，應使用此語句：

mysql> ALTER TABLE cs default CHARACTER SET utf8 collate utf8_general_ci,LOCK=NONE;         
Query OK,0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
 
mysql> select TABLE_SCHEMA,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,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)

可以發現列字符集沒有改變，並且只有新的列才會預設繼承表的字符集（utf8.utf8_general_ci）。

總結

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