【Leetcode Database】Duplicate Emails

題目：

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

+----+---------+
| Id | Email   |
+----+---------+
| 1  | [email protected] |
| 2  | [email protected] |
| 3  | [email protected] |
+----+---------+

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

+---------+
| Email   |
+---------+
| [email protected] |
+---------+

Note: All emails are in lowercase.

代碼：

1 2	# Write your MySQL query statement below select Email from Person group by Email having count(Email)>1

此題考察group by 的having以及count(列名)的用法

（1）group by 具體請看http://www.cnblogs.com/rainman/archive/2013/05/01/3053703.html（這裏講的很詳細）

（2）count(列名)是對列中不爲空的行進行計數，如果某列有n個記錄，m(m<=n)個記錄爲null，則count（該列）=n-m。