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.

 Naive Way: 我先建一个Table(Email, count(*)), 然后在这个Table里找count(*) > 1的。

 SELECT Email  
 FROM   
 (SELECT Email, count(*) AS num  
 FROM Person  
 FROUP BY Email) E  
 WHERE num > 1;  

Improved Way: 不需要使用多余的subquery。

 SELECT Email from Person  
 GROUP BY Email  
 HAVING count(Email)>1;