Mysql实现"查找重复的电子邮箱"的两种方法

写SQL语句查询Person表中所有重复的电子邮箱

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

例如，你的语句对上表应该返回如下结果

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

注意：所有的emails都是小写

1：先按邮箱Email分组，生成临时表A，表A中有Email和对应出现的次数。然后判断次数是否大于1

SELECT Email
FROM (SELECT Email, COUNT(*) AS Sum
      FROM Person
     GROUP BY Email) AS A
WHERE A.Sum>1

2：利用HAVING

WHERE 关键字无法与聚合函数一起使用。HAVING 子句可以让我们筛选分组后的各组数据。

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

算法题来自：https://leetcode-cn.com/problems/duplicate-emails/description/