共计 2633 个字符,预计需要花费 7 分钟才能阅读完成。
MySQL 出现 You can’t specify target table for update in FROM clause 这个错误的意思是 不能在同一个 sql 语句中,先 select 同一个表的某些值,然后再 update 这个表。
例如:message 表保存了多个用户的消息
创建表
CREATE TABLE `message` (`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`uid` int(10) unsigned NOT NULL,
`content` varchar(255) NOT NULL,
`addtime` datetime NOT NULL,
PRIMARY KEY (`id`),
KEY `uid` (`uid`),
KEY `addtime` (`addtime`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
插入数据
insert into message(uid,content,addtime) values
(1,'content1','2016-09-26 00:00:01'),
(2,'content2','2016-09-26 00:00:02'),
(3,'content3','2016-09-26 00:00:03'),
(1,'content4','2016-09-26 00:00:04'),
(3,'content5','2016-09-26 00:00:05'),
(2,'content6','2016-09-26 00:00:06'),
(2,'content7','2016-09-26 00:00:07'),
(4,'content8','2016-09-26 00:00:08'),
(4,'content9','2016-09-26 00:00:09'),
(1,'content10','2016-09-26 00:00:10');
表结构及数据如下:
mysql> select * from message;
+----+-----+-----------+---------------------+
| id | uid | content | addtime |
+----+-----+-----------+---------------------+
| 1 | 1 | content1 | 2016-09-26 00:00:01 |
| 2 | 2 | content2 | 2016-09-26 00:00:02 |
| 3 | 3 | content3 | 2016-09-26 00:00:03 |
| 4 | 1 | content4 | 2016-09-26 00:00:04 |
| 5 | 3 | content5 | 2016-09-26 00:00:05 |
| 6 | 2 | content6 | 2016-09-26 00:00:06 |
| 7 | 2 | content7 | 2016-09-26 00:00:07 |
| 8 | 4 | content8 | 2016-09-26 00:00:08 |
| 9 | 4 | content9 | 2016-09-26 00:00:09 |
| 10 | 1 | content10 | 2016-09-26 00:00:10 |
+----+-----+-----------+---------------------+
10 rows in set (0.00 sec)
然后执行将每个用户第一条消息的内容更新为 Hello World
mysql> update message set content='Hello World' where id in(select min(id) from message group by uid);
ERROR 1093 (HY000): You can't specify target table'message' for update in FROM clause
因为在同一个 sql 语句中,先 select 出 message 表中每个用户消息的最小 id 值,然后再更新 message 表,因此会出现 ERROR 1093 (HY000): You can’t specify target table‘message’for update in FROM clause 这个错误。
解决方法:select 的结果再通过一个中间表 select 多一次,就可以避免这个错误
update message set content='Hello World' where id in(select min_id from (select min(id) as min_id from message group by uid) as a );
执行:
mysql> update message set content='Hello World' where id in(select min_id from (select min(id) as min_id from message group by uid) as a );
Query OK, 4 rows affected (0.01 sec)
Rows matched: 4 Changed: 4 Warnings: 0
mysql> select * from message;
+----+-----+-------------+---------------------+
| id | uid | content | addtime |
+----+-----+-------------+---------------------+
| 1 | 1 | Hello World | 2016-09-26 00:00:01 |
| 2 | 2 | Hello World | 2016-09-26 00:00:02 |
| 3 | 3 | Hello World | 2016-09-26 00:00:03 |
| 4 | 1 | content4 | 2016-09-26 00:00:04 |
| 5 | 3 | content5 | 2016-09-26 00:00:05 |
| 6 | 2 | content6 | 2016-09-26 00:00:06 |
| 7 | 2 | content7 | 2016-09-26 00:00:07 |
| 8 | 4 | Hello World | 2016-09-26 00:00:08 |
| 9 | 4 | content9 | 2016-09-26 00:00:09 |
| 10 | 1 | content10 | 2016-09-26 00:00:10 |
+----+-----+-------------+---------------------+
10 rows in set (0.00 sec)
注意,只有 mysql 会有这个问题,mssql 与 Oracle 都没有这个问题。
本文永久更新链接地址:http://www.linuxidc.com/Linux/2017-02/140209.htm
正文完
星哥玩云-微信公众号