第五题：Duplicate Emails

https://leetcode.com/problems/delete-duplicate-emails/description/

邮件地址表：

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

查找重复的邮件地址：

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

SELECT Email
FROM Person
GROUP BY (Email) HAVING COUNT(Id) >=2;