题解 | #牛客直播各科目同时在线人数#
牛客直播各科目同时在线人数
https://www.nowcoder.com/practice/d69677e41f9a4bf3b3ed7a42573e9490
select course_id,course_name,max(num)
from(
select course_id,time,situation,
sum(situation) over(partition by course_id order by time rows between unbounded preceding and current row) num
from (
select user_id,course_id,in_datetime time,1 situation from attend_tb
union
select user_id,course_id,out_datetime time ,-1 situation from attend_tb
) UN
) num
left join course_tb using(course_id)
group by course_name,course_id
