力扣数据库 - 182.查找重复的电子邮箱

SQL架构

Create table If Not Exists Person (Id int, Email varchar(255))
Truncate table Person
insert into Person (Id, Email) values ('1', '[email protected]')
insert into Person (Id, Email) values ('2', '[email protected]')
insert into Person (Id, Email) values ('3', '[email protected]')

题目描述

编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

+----+---------+
| Id | Email   |
+----+---------+
| 1  | [email protected] |
| 2  | [email protected] |
| 3  | [email protected] |
+----+---------+

根据以上输入，你的查询应返回以下结果：

+---------+
| Email   |
+---------+
| [email protected] |
+---------+

说明：所有电子邮箱都是小写字母。

解法

方法1：使用GROUP BY + COUNT(*)

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

耗时：345ms

方法2：使用WHERE

SELECT DISTINCT a.Email
FROM Person a,Person b
WHERE a.Email=b.Email
AND a.Id<>b.Id;

耗时：404 ms