mysql 的delete from 子查询限制

我就是我 2022-07-12 09:45 462阅读 0赞

转自：http://blog.csdn.net/u013614451/article/details/41419471

最经做项目时发现的问题，好像在update时也有。。。

网上查到的资料如下：

1.使用[MySQL][]进行delete from操作时，若子查询的 FROM 字句和更新/删除对象使用同一张表，会出现错误。

mysql> DELETE FROM tab1 WHERE col1 = ( SELECT MAX( col1 ) FROM tab1 );   
ERROR 1093 (HY000): You can’t specify target table ‘tab1′ for update in FROM clause   
  
针对“同一张表”这个限制，撇开效率不谈，多数情况下都可以通过多加一层select 别名表来变通解决，像这样   
  
DELETE FROM tab1   
WHERE col1 = (   
SELECT MAX( col1 )   
FROM (   
SELECT \* FROM tab1   
) AS t   
);   
  
  
\------------------------------------------------------------------------   
  
2. mysql delete from where in 时后面 的查询语句里不能加where条件   
  
  
Sql代码   
      delete from \`t\_goods\` where fi\_id in (select \* from ( select fi\_id from \`t\_goods\` where fs\_num is null and fs\_name is null and fs\_type is null and fs\_using is null and fs\_lifetime is null) b)    
  
  
Sql代码   
      delete from \`t\_goods\` where fi\_id in (select fi\_id from \`t\_goods\` where fs\_num is null and fs\_name is null and fs\_type is null and fs\_using is null and fs\_lifetime is null)     
  
  
Sql代码   
      delete from \`t\_goods\` where fi\_id in ( select fi\_id from \`t\_goods\` )     
  
上面三种情况，只有中间的不能执行。   
  
综合起来就是mysql delete from where in 时后面 的查询语句里不能加where条件   
  
\---------------------------------------------------------------------------   
  
  
3. delete from table... 这其中table不能使用别名   
  
Sql代码   
  
    delete from student a where a.id in (1,2);(执行失败)   
  
      
  
     select a.\* from student a where a.id in (1,2);(执行成功)

# 二，mysql中You can’t specify target table for update in FROM clause错误解决方法 #

mysql中You can't specify target table for update in FROM clause错误的意思是说，不能先select出同一表中的某些值，再update这个表(在同一语句中)。 例如下面这个sql：

复制代码 代码如下:

delete from tbl where id in  
(  
        select max(id) from tbl a where EXISTS  
        (  
            select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1  
        )  
        group by tac  
)

改写成下面就行了：

复制代码 代码如下:

delete from tbl where id in  
(  
    select a.id from  
    (  
        select max(id) id from tbl a where EXISTS  
        (  
            select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1  
        )  
        group by tac  
    ) a  
)

也就是说将select出的结果再通过中间表select一遍，这样就规避了错误。注意，这个问题只出现于mysql，mssql和oracle不会出现此问题。

[MySQL]: http://lib.csdn.net/base/mysql