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.
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1 # Write your MySQL query statement below 2 select distinct p1.Email from 3 Person p1,Person p2 4 where p1.Email=p2.Email 5 and p1.Id!=p2.Id
本章讲解 SELECT DISTINCT 语句。
在表中，可能会包含重复值。这并不成问题，不过，有时您也许希望仅仅列出不同（distinct）的值。
关键词 DISTINCT 用于返回唯一不同的值。
SELECT DISTINCT 列名称 FROM 表名称
如果要从 "Company" 列中选取所有的值，我们需要使用 SELECT 语句：
SELECT Company FROM Orders
| Company | OrderNumber |
|---|---|
| IBM | 3532 |
| 3wschool | 2356 |
| Apple | 4698 |
| 3wschool | 6953 |
| Company |
|---|
| IBM |
| 3wschool |
| Apple |
| 3wschool |
请注意，在结果集中，3wschool 被列出了两次。
如需从 Company" 列中仅选取唯一不同的值，我们需要使用 SELECT DISTINCT 语句：
SELECT DISTINCT Company FROM Orders
| Company |
|---|
| IBM |
| 3wschool |
| Apple |
现在，在结果集中，"3wschool" 仅被列出了一次。
leetcode 182. Duplicate Emails
原文：http://www.cnblogs.com/njczy2010/p/5193373.html