[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。
_
