You can't specify target table for update in from clause error in MySQL

One 、 analysis

mysql appear You can’t specify target table for update in FROM clause This mistake means that it can't be in the same sql In the sentence , First select Some values of the same table , And then again update This table

Two 、 The phenomenon

for example ：message Table holds messages from multiple users 　
Create table

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 data

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');

Table structure and data are as follows ：

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)

What makes a mistake SQL
Then execute to update the content of the first message of each user to 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

Because in the same sql In the sentence , First select Out message Minimum number of messages per user in the table id value , Then update message surface , So there will be ERROR 1093 (HY000): You can’t specify target table ‘message’ for update in FROM clause This mistake .

3、 ... and 、 resolvent

select The results are then passed through an intermediate table select One more time , You can avoid this mistake

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 );

perform

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)

Be careful , Only mysql There will be this problem ,mssql And oracle No such problem .

Four 、 explain

When a request is reading data , Other requests can also be read , But I can't write , Because once another thread writes data , This will cause the data read by the current thread to be not the latest , This is the phenomenon of non repeatable reading . This limitation can be circumvented by using the form of generated tables , because MySQL This table will only be treated as a temporary table .

Link to the original text ：https://blog.csdn.net/fdipzone/article/details/52695371