LeetCode：Database 89.获取最近第二次的活动

Posted 2021-06-28 Xiao Miao

要求：写一条SQL查询展示每一位用户最近第二次的活动

如果用户仅有一次活动，返回该活动，一个用户不能同时进行超过一项活动，以 任意 顺序返回结果

表: UserActivity的结构

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| username      | varchar |
| activity      | varchar |
| startDate     | Date    |
| endDate       | Date    |
+---------------+---------+
该表不包含主键
该表包含每个用户在一段时间内进行的活动的信息
名为 username 的用户在 startDate 到 endDate 日内有一次活动

UserActivity 表:

+------------+--------------+-------------+-------------+
| username   | activity     | startDate   | endDate     |
+------------+--------------+-------------+-------------+
| Alice      | Travel       | 2020-02-12  | 2020-02-20  |
| Alice      | Dancing      | 2020-02-21  | 2020-02-23  |
| Alice      | Travel       | 2020-02-24  | 2020-02-28  |
| Bob        | Travel       | 2020-02-11  | 2020-02-18  |
+------------+--------------+-------------+-------------+

Result TAble：

+------------+--------------+-------------+-------------+
| username   | activity     | startDate   | endDate     |
+------------+--------------+-------------+-------------+
| Alice      | Dancing      | 2020-02-21  | 2020-02-23  |
| Bob        | Travel       | 2020-02-11  | 2020-02-18  |
+------------+--------------+-------------+-------------+

Alice 最近一次的活动是从 2020-02-24 到 2020-02-28 的旅行, 在此之前的 2020-02-21 到 2020-02-23 她进行了舞蹈
Bob 只有一条记录，我们就取这条记录

SQL语句：

select username,activity,startDate,endDate from (    
select username,activity,startDate,endDate,row_number() over(partition by username order by startDate desc) as r,count(*) over(partition by username) as c1 
from UserActivity)a 
where if(c1>1,r=2,r=1);