[MySQL]LeetCode196 Delete Duplicate Emails

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  |
+----+------------------+

题意:将一个表中重复的email行去掉只保留一行，且保留的Id最小。

解法一：

delete from Person where Id not in
(select min_id from
(select min(id) as min_id from Person group by Email)
 as tmp);

解法二：

delete p1 from Person p1 inner join Person p2 where p1.Email=p2.Email and p1.Id>p2.Id  //内连接将重复的email找出来，再删除

有人看了上面解法一的答案：会问select min_id from这句话好像没啥用。得到的不还是内层select的结果么？但是我去掉了出现如下问题。

You can't specify target table 'Person' for update in FROM clause 意思是，你不能在from的子句中，指定被用来更新的目标表。也就是在Mysql中在同一个表中不能select之后再update.所以将select出的内容作为tmp临时表，然后从tmp中选出min_id再删除。