MYSQL-DELETE

[https://leetcode.com/problems/delete-duplicate-emails/]
MYSQL的DETETE语句，包含子查询时，如果子查询与被删除数据的表是一张表，会出错。像下面这样是不行的。

DELETE FROM person 
WHERE id NOT IN (
SELECT MIN(Id) 
FROM person
GROUP BY Email
)

解决办法是在子查询语句里再嵌套一个子查询，像下面这样。

DELETE FROM person 
WHERE id NOT IN ( 
SELECT MIN(Id) 
FROM ( 
SELECT * FROM person 
) AS t 
GROUP BY t.Email
);

然而，这样就超时了，效率太低，因为子查询是SELECT * FROM person，直接把全表拿来了，没做任何处理。较好的做法如下：

DELETE FROM Person
WHERE Id NOT IN (SELECT Id 
           FROM 
            (SELECT MIN(Id) AS Id 
             FROM Person 
             GROUP BY Email
            ) p
          );

这么做的话最里面的子查询查出的结果集会比较小，外面的筛选会快很多。
^_