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.
本题是找表Person中重复的邮箱，并且该表中的邮箱字母都是小写字母。这道题比较简单，主要涉及到的知识点就是group by 与 having count 的组合，具体的解法如下：
解法一：
SELECT Email from person GROUP BY Email HAVING COUNT(Email)>1;
解法二：
我们可以使用内交来做，用Email来内交两个表，然后返回Id不同的行，则说明两个不同的人使用了相同的邮箱：
SELECT DISTINCT p1.Email FROM person p1 JOIN person p2 on p1.Email = p2.Email WHERE p1.Id <> p2.Id;