[https://leetcode.com/problems/delete-duplicate-emails/]
MYSQL的DETETE語句，包含子查詢時，如果子查詢與被刪除數(shù)據(jù)的表是一張表，會出錯。像下面這樣是不行的。

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

解決辦法是在子查詢語句里再嵌套一個子查詢，像下面這樣。

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

然而，這樣就超時了，效率太低，因為子查詢是SELECT * FROM person，直接把全表拿來了，沒做任何處理。較好的做法如下：

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

這么做的話最里面的子查詢查出的結果集會比較小，外面的篩選會快很多。
^_