04-Duplicate-Emails.md

Duplicate Emails

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]

解法

容易想到group by和子查询

SELECT Email FROM (
  SELECT Email, count(Email) AS num FROM Person GROUP BY Email
) a WHERE a.num > 1

参考答案还有一个HAVEING的用法

SELECT Email FROM Person GROUP BY Email HAVING count(Email) > 1