• 注意对比时间是19:00:00，不能直接用in_datetime和out_datetime和“2021-12-01 19:00:00”来比较，为什么呢
select 
course_id,
any_value(course_name),
count(user_id) as online_num
from attend_tb 
join course_tb using(course_id)
where time(in_datetime)<='19:00:00'
and time(out_datetime)>='19:00:00'
group by course_id
order by course_id