题解 | 牛客直播各科目同时在线人数

with a as(select user_id, course_id, in_datetime as time, 1 as uv
from attend_tb
union all
select user_id, course_id, out_datetime as time, -1 as uv
from attend_tb),
b as(select *, sum(uv)over(partition by course_id order by time) as zaixian
from a
order by course_id, time)
select b.course_id, c.course_name, max(b.zaixian) max_num
from b left join course_tb c on b.course_id=c.course_id
group by 1,2