Leetcode数据库刷题

1. 查找重复的电子邮箱

自己想法（错误）

SELECT a.Email
FROM Person a,Person b
WHERE a.Email = b.Email

结果：表内全部出现
解决思路：计算每封邮件的存在次数

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

解二：创建临时表

SELECT Email
FROM(
 SELECT Email,COUNT(Email) as num
 FROM Person
 GROUP BY Email
) tmp_table~~~~
WHERE num>1