【sql】删除重复的邮件 Delete Duplicate Emails

问题：

Write a SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its smallest Id.

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+
Id is the primary key column for this table.

For example, after running your query, the above Person table should have the following rows:

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
+----+------------------+

解决：

① 能够首先找出全部不重复的邮箱，而后取个反就是重复的邮箱，都删掉便可，那么咱们如何找出全部不重复的邮箱呢，咱们能够按照邮箱群组起来，而后用Min关键字挑出较小的，而后取补集删除便可。962ms

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

② 使用内交让两个表以邮箱关联起来，而后把相同邮箱且Id大的删除掉；954ms

DELETE p2 FROM Person p1 JOIN Person p2
ON p2.Email = p1.Email WHERE p2.Id > p1.Id;

③ 直接用where将两表关联起来；1444ms。

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