题解 | #牛客直播各科目同时在线人数#

SELECT c.course_id,course_name,max_num
FROM course_tb d,(SELECT course_id,max(num)as max_num
from(
select course_id,
sum(diff) over (partition by course_id order by dt,diff desc)as num
from
(
SELECT course_id,in_datetime as dt,1 as diff
from attend_tb
union all 
SELECT course_id,out_datetime as dt,-1 as diff
from attend_tb)a)b
group by course_id)c
where d.course_id=c.course_id
order by c.course_id