首先创建一个表:
CREATE TABLE `t1` (
`id` INT(11) NULL DEFAULT NULL,
`name` VARCHAR(20) NULL DEFAULT NULL
)
插入几条数据:
mysql> select * from t1;
+------+------+
| id | name |
+------+------+
| 1 | chen |
| 2 | li |
| 3 | huan |
+------+------+
3 rows in set (0.00 sec)
需求1:删除最大id的那条记录,于是我们会大约写出如下的语句:
mysql> delete from t1 where id=(select max(id) from t1);
ERROR 1093 (HY000): You can't specify target table 't1' for update in FROM clause
很不幸,它报错了.
可以修改成如下语句:
mysql> delete a from t1 a,(select max(id) maxid from t1) b where a.id=b.maxid;
Query OK, 1 row affected (0.01 sec) mysql> select * from t1;
+------+------+
| id | name |
+------+------+
| 1 | chen |
| 2 | li |
+------+------+
2 rows in set (0.00 sec)
也可以是如下语句:
mysql> delete from t1 where id in ( select a.maxid from (select max(id) maxid from t1) a);
Query OK, 1 row affected (0.01 sec) mysql> select * from t1;
+------+------+
| id | name |
+------+------+
| 1 | chen |
+------+------+
1 row in set (0.00 sec)
需求2:插入一条记录,并且id值是之前该表最大值加1,于是我们会大约写出如下的语句:
mysql> insert into t1 values( (select max(id)+1 from t1),'you');
ERROR 1093 (HY000): You can't specify target table 't1' for update in FROM clause
依旧报了同样的错误
可以改写如下:
mysql> insert into t1 select (select max(id)+1 maxid from t1 ) , 'you';
Query OK, 1 row affected (0.06 sec)
Records: 1 Duplicates: 0 Warnings: 0 mysql> select * from t1;
+------+------+
| id | name |
+------+------+
| 1 | chen |
| 2 | you |
+------+------+
2 rows in set (0.00 sec)
需求3:我们要更新一条语句,id需要变为之前最大值加1,于是我们会大约写出如下的语句:
mysql> update t1 set id=(select max(id)+1 from t1) where id=1;
ERROR 1093 (HY000): You can't specify target table 't1' for update in FROM clause
错误如初
我们可以改写为如下语句:
mysql> update t1,(select max(id)+1 as maxid from t1 ) a set id=a.maxid where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t1;
+------+------+
| id | name |
+------+------+
| 3 | chen |
| 2 | you |
+------+------+
2 rows in set (0.00 sec)
也可以改成如下语句:
mysql> update t1 set id=(select a.maxid from (select max(id)+1 maxid from t1) a) where id=3;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t1;
+------+------+
| id | name |
+------+------+
| 4 | chen |
| 2 | you |
+------+------+
2 rows in set (0.00 sec)
总的思路是:把查询的最大值语句转为subquery或derived。