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。
1.groud by + having
select email from person GROUP BY email having count(*) > 1;
2.groud by + 临时表
select Email from
(
select Email, count(Email) as num
from Person
group by Email
) as statistic
where num > 1
;