JAVA高级面试进阶训练营视频教程

admin

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。