编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

示例：

+----+---------+
| Id | Email |
+----+---------+
| 1 | a@b.com |
| 2 | c@d.com |
| 3 | a@b.com |
+----+---------+

根据以上输入，你的查询应返回以下结果：

+---------+
| Email |
+---------+
| a@b.com |
+---------+

说明：所有电子邮箱都是小写字母。

 

思路一

# Write your MySQL query statement below
Select Email from Person group by Email having count(Email) > 1

 

备注：

优先顺序。where>group by>having>order by

where 在 group by 之前执行，having 在 group by 之后执行。如果上面把 having 改为 where ，就会产生 group by 还没有分组完，就执行了计数。

 

思路二

# Write your MySQL query statement below
Select Email from (Select Email , count(Email) as num from Person group by Email) as statistic  where num > 1

 

备注：

以 Email 分组，以 Email 个数作为 num 列。然后以此当做辅助表 statistic ，从辅助表里取 num > 1 的，筛选出重复的