leetcode Database2

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一、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.

分析：编写一个SQL查询从Person表中找出所有重复的邮箱地址。

解法一：（self join）

# Write your MySQL query statement below
select distinct a.Email from Person a, Person b where a.Email=b.Email and a.Id<>b.Id

            一开始，写的时候没注意把distinct给漏了，导致出错：

            Submission Result: Wrong AnswerMore Details 

解法二：

# Write your MySQL query statement below
Select Email
From Person
GROUP BY Email
Having count(Email)>1

leetcode Database2

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原文地址：http://www.cnblogs.com/carsonzhu/p/4630352.html