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  | [email protected] |
| 2  | [email protected]  |
| 3  | [email protected]
 
 |
+----+------------------+
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  | [email protected] |
| 2  | [email protected]
 
  |
+----+------------------+

題意:將一個表中重複的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  //內連線將重複的email找出來，再刪除

有人看了上面解法一的答案：會問select min_id from這句話好像沒啥用。得到的不還是內層select的結果麼？但是我去掉了出現如下問題。

You can't specify target table 'Person' for update in FROM clause 意思是，你不能在from的子句中，指定被用來更新的目標表。也就是在Mysql中在同一個表中不能select之後再update.所以將select出的內容作為tmp臨時表，然後從tmp中選出min_id再刪除。