Mysql的一些操作（删除表中的重复元素，查询重复元素）

问题1：去除person表中的重复元素（如下图所示）



正确答案如下代码一：

delete from person where
pname in (select a.pn from
(select pname pn from person group by pname having count(pname) > 1)a
)
and
pid not in (select b.pi from
(select min(pid) pi from person group by pname having count(pname) > 1)b
)

结果如下：



--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

一般有人会这样写，但会出错，如下图1-1所示

delete from person where
pname in (select pname pn from person group by pname having count(pname) > 1)
and
pid not in (select min(pid) pi from person group by pname having count(pname) > 1)

图1-1 失败代码

这样写会报错：[Err] 1093 - You can't specify target table 'person' for update in FROM clause（意思是不能先select出同一表中的某些值，再update这个表(在同一语句中)。）

所以如代码一所示，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里面，SqlServer和oracle不会出现此问题。

代码二：多字段删除重复的

如下表中数据：





执行代码如下：

delete from person where
(pname,pvalue) in (select a.pn,a.pv from
(select pname pn,pvalue pv from person group by pname,pvalue having count(*) > 1)a
)
and
pid not in (select b.pi from
(select min(pid) pi from person group by pname,pvalue having count(*) > 1)b
)

结果图：



问题二：查询表中的重复元素

select pname from person group by pname having count(pname) > 1

显示所有的重复元素：

select * from person where pname in (select pname from person group by pname having count(pname) > 1)