leetcode182-Duplicate Emails(找出出现重复的数据)

问题描述：

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 |
+---------+

问题求解：

1、distinct

select distinct p1.email from Person p1, Person p2
where p1.email = p2.email and p1.Id != p2.Id

2、group by & having

select email from Person
group by email
having count(*)>1