[SQL] LeetCode 182. 查找重复的电子邮箱 - [Easy]

182. 查找重复的电子邮箱

题目：

编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

示例：

+----+---------+
| 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。

---

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]');

---

解题语句：

方法一：单表查询：GROUP BY + HAVING

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

• 注：COUNT( * ) 比 COUNT( Email ) 效率高

方法二：子查询，创建临时表作为数据源

SELECT
	Email 
FROM
	( SELECT Email, COUNT( Email ) AS c FROM Person GROUP BY Email ) AS p 
WHERE
	c > 1;

方法三：自关联

SELECT DISTINCT
	p1.Email AS Email 
FROM
	Person AS p1
	INNER JOIN Person AS p2 ON p1.Id != p2.Id 
	AND p1.Email = p2.Email;

效率上，单表查询 优于 关联查询 优于 子查询。