182. 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.

 

这道题让我们求重复的邮箱,那么最直接的方法就是用Group by…Having Count(*)…的固定搭配来做,代码如下:

 

解法一:

SELECT Email FROM Person GROUP BY Email
HAVING COUNT(*) > 1;

 

我们还可以用内交来做,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:

 

解法二:

SELECT DISTINCT p1.Email FROM Person p1
JOIN Person p2 ON p1.Email = p2.Email
WHERE p1.Id <> p2.Id;

 

参考资料:

https://leetcode.com/discuss/53206/a-solution-using-a-group-by-and-another-one-using-a-self-join

 

LeetCode All in One 题目讲解汇总(持续更新中…)


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