README.md

Duplicate Emails

Problem

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.

Solution 1

直接使用子查询

Code

select Email
from (select Email, count(*)  count from Person  group by Email) as e
where e.count > 1;

Solution 2

分组过滤

Code

select Email
from Person group by Email
having count(Email) > 1;