题目
编写一个 SQL 查询，来删除 Person 表中所有重复的电子邮箱，重复的邮箱里只保留 Id 最小 的那个。
+----+------------------+
| Id | Email |
+----+------------------+
| 1 | john@example.com |
| 2 | bob@example.com |
| 3 | john@example.com |
+----+------------------+
Id 是这个表的主键。
例如，在运行你的查询语句之后，上面的 Person 表应返回以下几行:
+----+------------------+
| Id | Email |
+----+------------------+
| 1 | john@example.com |
| 2 | bob@example.com |
+----+------------------+
答案分析
注意没有加t会出现报错信息：
You can’t specify target table ‘tempA’ for update in FROM clause
需要查询的时候增加一层中间表，就可以避免该错误。
我的答案：
# Write your MySQL query statement below
delete from Person
where Id not in
(
select Id from
(
select min(Id) as Id from Person group by Email having count(*) > 1
) t
)
and
Email in
(
select Email from
(
select Email from Person group by Email having count(*) > 1
) t
)
别人的答案：
delete from Person
where id not in
(select m.mi from
(select min(id) mi from Person group by email) as m
)