难度：简单
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 | [email protected] |
| 2 | [email protected] |
| 3 | [email protected] |
+----+---------+
根据以上输入，你的查询应返回以下结果：
+---------+
| Email |
+---------+
| [email protected] |
+---------+
解答
方法一： 看到“查重复”这样的题，首先要用分组函数（group by），再用聚合函数count给email计数直接做判断
select Email
From Person
group by Email
having count(Email)>=2
方法二： 还有一个使用临时表的方式，稍稍有点麻烦
select Email, count(Email) as num
from Person
group by Email
将上表作为临时表，查出不同的email有的个数，然后再判断num>=2所对应的Email
具体sql语句如下：
select Email from
(
select Email, count(Email) as num
from Person
group by Email
) as statistic
where num >= 1