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 | a@b.com |
| 2 | c@d.com |
| 3 | a@b.com |
+----+---------+
根据以上输入,你的查询应返回以下结果:
+---------+
| Email |
+---------+
| a@b.com |
+---------+
说明:所有电子邮箱都是小写字母。
`解题思路`
1 嵌套表查询
`select Email from (
select Email, count(Email) as a
from Person
group by Email) as b
where a>1`
2 group by ... having
`select Email
from Person
group by Email
having count(Email) > 1;`
- where 子句的作用是在对查询结果进行分组前,将不符合where条件的行去掉,即在分组之前过滤数据,where条件中不能包含聚组函数,使用where条件过滤出特定的行。
- having 子句的作用是筛选满足条件的组,即在分组之后过滤数据,条件中经常包含聚组函数,使用having 条件过滤出特定的组,也可以使用多个分组标准进行分组。