题解 | #牛客直播各科目同时在线人数#

select course_id, course_name, max(so) as max_num
from (
    select course_id,sum(uv) over(partition by course_id order by dt) as so
    from (
        select course_id, user_id, in_datetime as dt, 1 as uv
        from attend_tb
        union 
        select course_id, user_id, out_datetime as dt, -1 as uv
        from attend_tb
    ) a
) b join course_tb using(course_id)
group by course_id, course_name
order by course_id