mysql Error1093错误

prollerweg

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