文巾解题 196. 删除重复的电子邮箱

Posted 2021-08-01 刘文巾

1 题目描述

2 解题思路

2.1 双表联立

DELETE p1 
FROM Person p1, Person p2
WHERE p1.Email = p2.Email 
      AND p1.Id > p2.Id

把相同邮箱，大的都去掉

2.2 使用子查询

用子查询的分组操作，找到每一个email的最小id号，然后将不是最小id号的删除

DELETE p1 
FROM Person p1
WHERE p1.Id NOT IN (
        SELECT(a.id) 
        FROM(SELECT MIN(P2.Id) AS id
            FROM PERSON P2
            GROUP bY P2.Email) a)

这道题卡了一段时间，一开始我是这么写的：

DELETE p1 
FROM Person p1
WHERE p1.Id NOT IN (
        SELECT MIN(P2.Id) AS id
            FROM PERSON P2
            GROUP bY P2.Email)

然后报错“You can't specify target table 'p1' for update in FROM clause“

什么意思呢？就是我们在DELETE Person表的同时，对Person表进行了子查询，SELECT筛选操作。

解决方法就像上面一样，我们再套一层。这样进行DELETE和筛选的表就不一样了