Duplicate Emails

Write a SQL query to find all duplicate emails in a table named Person.

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

For example, your query should return the following for the above table:

+---------+
| Email   |
+---------+
| a@b.com |
+---------+
Note: All emails are in lowercase.
用exists

 select distinct Email from Person p1 where exists (select Id from Person p2 where p1.Email = p2.Email and p1.Id != p2.Id);

用group by

 select Email from Person group by Email having count(Email) > 1;