用过Oracle和MySQL的朋友，对其中使用上的一些区别，就会比较敏感，例如字符集，就算其中一个，Oracle除了建库会指定字符集外，好像很少提到了，而MySQL中创建表的时候可能都会指定字符集，还可能导致出现隐式转换（可参考《MySQL隐式转换案例一则》《Oracle、SQL Server和MySQL的隐式转换异同》），都需要格外关注。碰巧看到社区退的这篇文章《故障分析 | MySQL convert 函数导致的字符集报错处理》，了解一下函数导致的字符集报错问题。
一、问题背景
有客户之前遇到一个MySQL 8.0.21实例中排序规则的报错，是在调用视图时抛出，报错信息如下，
代码语言：javascript复制ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='二、问题模拟
代码语言：javascript复制mysql> show create table t1G;
*************************** 1. row ***************************
Table: t1
Create Table: CREATE TABLE `t1` (
`name1` varchar(12) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci
1 row in set (0.00 sec)
mysql> show create table t2G;
*************************** 1. row ***************************
Table: t2
Create Table: CREATE TABLE `t2` (
`name2` varchar(12) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
mysql> CREATE VIEW t3 as select * from t1,t2 where `t1`.`name1`= `t2`.`name2`;
Query OK, 0 rows affected (0.06 sec)
mysql> select * from t3;
ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='三、问题分析
通过查看视图定义，可以发现由于视图中涉及到的两张表字符集不同，所以创建视图时MySQL会自动使用convert函数转换字符集，
代码语言：javascript复制mysql> show create view t3G;
*************************** 1. row ***************************
View: t3
Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `t3` AS select `t1`.`name1` AS `name1`,`t2`.`name2` AS `name2` from (`t1` join `t2`) where (`t1`.`name1` = convert(`t2`.`name2` using utf8mb4))
character_set_client: utf8mb4
collation_connection: utf8mb4_general_ci
1 row in set (0.00 sec)在MySQL 8.0中utf8mb4的默认排序规则为utf8mb4_0900_ai_ci，而在t1表的排序规则为utf8mb4_general_ci，那么我们试着将排序规则相关的参数修改后再执行SQL看看，修改后的环境参数如下，
代码语言：javascript复制mysql> show variables like '%collat%';
------------------------------- --------------------
| Variable_name | Value |
------------------------------- --------------------
| collation_connection | utf8mb4_general_ci |
| collation_database | utf8mb4_bin |
| collation_server | utf8mb4_bin |
| default_collation_for_utf8mb4 | utf8mb4_general_ci |
------------------------------- -------------------- 再次执行SQL发现还是会报一样的错，
代码语言：javascript复制mysql> select * from t1,t2 where `t1`.`name1`=convert(`t2`.`name2` using utf8mb4);
ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='通过show collation来查看utf8mb4字符集对应的默认排序规则，输出显示默认规则为utf8mb4_general_ci，并不是utf8mb4_0900_ai_ci，
代码语言：javascript复制mysql> show collation like '%utf8mb4%';
---------------------------- --------- ----- --------- ---------- --------- ---------------
| Collation | Charset | Id | Default | Compiled | Sortlen | Pad_attribute |
---------------------------- --------- ----- --------- ---------- --------- ---------------
| utf8mb4_general_ci | utf8mb4 | 45 | Yes | Yes | 1 | PAD SPACE |
---------------------------- --------- ----- --------- ---------- --------- ---------------
mysql> show character set like '%utf8mb4%';
--------- --------------- -------------------- --------
| Charset | Description | Default collation | Maxlen |
--------- --------------- -------------------- --------
| utf8mb4 | UTF-8 Unicode | utf8mb4_general_ci | 4 |
--------- --------------- -------------------- --------
1 row in set (0.00 sec)继续排查发现元数据中的字符集默认排序规则如下，默认规则为utf8mb4_0900_ai_ci，
代码语言：javascript复制mysql> select * from INFORMATION_SCHEMA.COLLATIONS where IS_DEFAULT='Yes' and CHARACTER_SET_NAME='utf8mb4'G;
*************************** 1. row ***************************
COLLATION_NAME: utf8mb4_0900_ai_ci
CHARACTER_SET_NAME: utf8mb4
ID: 255
IS_DEFAULT: Yes
IS_COMPILED: Yes
SORTLEN: 0
PAD_ATTRIBUTE: NO PAD
1 row in set (0.00 sec)检查参数发现，元数据信息中utf8mb4字符集默认排序规则是utf8mb4_0900_ai_ci，show collation/show character输出的都是utf8mb4_general_ci。
为什么show显示的结果和INFORMATION_SCHEMA.COLLATIONS表查到的信息还不一样？
此处我们暂且按下不表，咱们先看看官方文档中convert函数用法，其中有下面这段原文，
If you specify CHARACTER SET charset_name as just shown, the character set and collation of the result are charset_name and the default collation of charset_name. If you omit CHARACTER SET charset_name, the character set and collation of the result are defined by the character_set_connection and collation_connection system variables that determine the default connection character set and collation (see Section 10.4, “Connection Character Sets and Collations”).
从上述原文可知如果convert只指定了字符集，那么该结果的排序规则就是所指定字符集的默认规则，由之前的测试情况可知，convert使用的是INFORMATION_SCHEMA.COLLATIONS的排序规则，而不是 default_collation_for_utf8mb4指定的utf8mb4_general_ci，那我们来看看 default_collation_for_utf8mb4参数主要作用场景，
(1)SHOW COLLATION and SHOW CHARACTER SET. (2)CREATE TABLE and ALTER TABLE having a CHARACTER SET utf8mb4 clause without a COLLATION clause, either for the table character set or for a column character set. (3)CREATE DATABASE and ALTER DATABASE having a CHARACTER SET utf8mb4 clause without a COLLATION clause. (4)Any statement containing a string literal of the form _utf8mb4'some text' without a COLLATE clause.
其中，第一点解释了为什么show查到的信息和元数据中信息不一样，default_collation_for_utf8mb4修改后影响show COLLATION and SHOW CHARACTER SET的查询结果，并不会改变字符集的默认排序规则，所以utf8mb4的默认规则还是utf8mb4_0900_ai_ci，SQL执行依然会报错。
将convert函数指定为t1.name1字段的排序规则后，SQL执行正常，
代码语言：javascript复制mysql> select * from t1,t2 where `t1`.`name1` = convert(`t2`.`name2` using utf8mb4) collate utf8mb4_general_ci;
------- -------
| name1 | name2 |
------- -------
| jack | jack |
------- -------
1 row in set (0.00 sec)另外，下面测试可以验证default_collation_for_utf8mb4的第四个场景，
代码语言：javascript复制mysql> select * from INFORMATION_SCHEMA.COLLATIONS where IS_DEFAULT='Yes' and CHARACTER_SET_NAME='utf8mb4'G;
*************************** 1. row ***************************
COLLATION_NAME: utf8mb4_0900_ai_ci
CHARACTER_SET_NAME: utf8mb4
ID: 255
IS_DEFAULT: Yes
IS_COMPILED: Yes
SORTLEN: 0
PAD_ATTRIBUTE: NO PAD
1 row in set (0.00 sec)
mysql> show variables like '�fault_collation%';
------------------------------- --------------------
| Variable_name | Value |
------------------------------- --------------------
| default_collation_for_utf8mb4 | utf8mb4_general_ci |
------------------------------- --------------------
1 row in set (0.01 sec)
mysql> set @s1 = _utf8mb4 'jack',@s2 = _utf8mb4 'jack';
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT @s1 = @s2;
-----------
| @s1 = @s2 |
-----------
| 1 |
-----------
1 row in set (0.00 sec)
_utf8mb4声明的@s1和@s2排序规则是default_collation_for_utf8mb4参数值，为utf8mb4_general_ci
mysql> SELECT @s1 = CONVERT(@s2 USING utf8mb4);
ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='
此时，经过CONVERT函数处理的@s2排序规则是utf8mb4_0900_ai_ci，所以会报错
mysql> SELECT @s1 = CONVERT(@s2 USING utf8mb4) collate utf8mb4_general_ci;
-------------------------------------------------------------
| @s1 = CONVERT(@s2 USING utf8mb4) collate utf8mb4_general_ci |
-------------------------------------------------------------
| 1 |
-------------------------------------------------------------
1 row in set (0.00 sec)四、问题回顾
运维中为避免字符集引起的报错问题，有如下建议可供参考：（具体参数值根据业务需求选择）
创建数据库实例时需指定参数character_set_database（默认值：utf8mb4），character_set_server（默认值：utf8mb4）。
当需要创建非默认字符集database/table时，需要在SQL中明确指定字符集和排序规则。
使用convert函数转换字符集时，当字段排序规则不是转换后字符集的默认排序规则，需要指定具体的排序规则。
代码语言：javascript复制SELECT @s1 = CONVERT(@s2 USING utf8mb4) collate utf8mb4_general_ciMySQL 5.7迁移至MySQL 8.0时，需注意MySQL 5.7版本中utf8mb4默认排序规则是utf8mb4_general_ci，MySQL 8.0中utf8mb4默认排序规则是utf8mb4_0900_ai_ci。
参考资料，
https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html https://dev.mysql.com/doc/refman/8.0/en/charset-connection.html https://dev.mysql.com/doc/refman/8.0/en/cast-functions.html#function_convert
