mysql出現You can’t specify target table for update in FROM clause 這個錯誤的意思是不能在同一個sql語句中，先select同一個表的某些值，然後再update這個表。
例如：message表保存了多個用戶的消息
創建表
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;
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');
表結構及數據如下：
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)
然後執行將每個用戶第一條消息的內容更新爲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
1
2
因爲在同一個sql語句中，先select出message表中每個用戶消息的最小id值，然後再更新message表，因此會出現 ERROR 1093 (HY000): You can’t specify target table ‘message’ for update in FROM clause 這個錯誤。
解決方法：select的結果再通過一箇中間表select多一次，就可以避免這個錯誤
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 );
1
執行：
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會有這個問題，mssql與oracle都沒有這個問題。