题解 | #牛客直播各科目同时在线人数#

请大家帮我看看哪里可以优化的~
with a as (select user_id,course_id,in_datetime dt,1 as uv from attend_tb
union all
select user_id,course_id,out_datetime dt,-1 as uv from attend_tb)

select course_id,course_name,max(num)
from
(select b.course_id,b.num,c.course_name
from
    (select course_id,sum(uv) over (partition by course_id order by dt) num
from a )b
left join course_tb c
on b.course_id=c.course_id)d
group by course_id,course_name
order by course_id