场景演示：
mysql> create table test(id int,name varchar(12),primary key(id));
Query OK, 0 rows affected (0.04 sec)
mysql> insert into test values(1,'ttt');
Query OK, 1 row affected (0.00 sec)
mysql> insert into test values(2,'ttt');
Query OK, 1 row affected (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set name='aaaa' where id in (select id from test);
ERROR 1093 (HY000): You can't specify target table 'test' for update in FROM clause官方说明：
Error: 1093 SQLSTATE: HY000 (ER_UPDATE_TABLE_USED)
Message: You can't specify target table '%s' for update in FROM clause
This error occurs for attempts to select from and modify the same table within a single statement. If the select attempt occurs within a derived table, you can avoid this error by setting the derived_merge flag of the optimizer_switch system variable to force the subquery to be materialized into a temporary table, which effectively causes it to be a different table from the one modified. See Section 9.2.2.3, “Optimizing Derived Tables and View References”.
mysql不支持，在一条语句对同一个表，先查询再更新的操作。
解决方法1：改成sql
比如：利用临时表
mysql> update test set name='aaaa' where id in (select id from (select id from test)c);
Query OK, 2 rows affected (0.02 sec)
Rows matched: 2 Changed: 2 Warnings: 0解决方法2：
官方说：可以通过设置optimizer_switch的 derived_merge参数来解决。
备注：Oracle是不会这个问题的
