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(guī)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，并且以_ci（大小寫不敏感）、_cs（大小寫敏感）或_bin（二進制）結(jié)束。在做比較時，應(yīng)該確保兩個表的字符排序相同。一般建表的時候不指定，可以走默認的，全是默認的就沒什么問題了。
下面來模擬一下各種場景，表結(jié)構(gòu)如下（utf8默認排序規(guī)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)
查看表默認排序規(guī)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)
查看列排序規(guī)則集
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)
從utf8升級為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.
從utf8.utf8_general_ci變更為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.
如果使用下面這種方式修改字符集，你會發(fā)現(xiàn)，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_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,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)
要僅僅改變一個表的默認字符集，應(yīng)使用此語句：
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)
可以發(fā)現(xiàn)列字符集沒有改變，并且只有新的列才會默認繼承表的字符集（utf8.utf8_general_ci）。
總結(jié)
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