题解 | #牛客直播开始时各直播间在线人数#

select 
    course_id,course_name,count(*) as online_num 
from (select 
    course_id,course_name,user_id
from (select 
    c.course_id,
    c.course_name,left(c.course_datetime,15) as start_time,
    a.user_id,a.in_datetime as intime
from course_tb c join attend_tb a on
c.course_id = a.course_id) t where intime <= start_time) p
group by course_name,course_id order by course_id;