题解 | #牛客直播开始时各直播间在线人数#

select a.course_id course_id
,t1.course_name course_name
,sum(a.uv) online_num
from
(
	select t3.course_id course_id
	,in_datetime dt
	,1 uv
	from attend_tb t3

	union all

	select t3.course_id course_id
	,out_datetime dt
	,-1 uv
	from attend_tb t3
)a
left join course_tb t1 on t1.course_id = a.course_id
where DATE_FORMAT(a.dt,'%T') < '19:01:00'
group by 1,2
order by 1