题解 | #牛客直播开始时各直播间在线人数#

select course_id, course_name, count(in_datetime)
from attend_tb left join course_tb using(course_id)
where '190000' between date_format(in_datetime,'%H%i%s') and date_format(out_datetime,'%H%i%s')
group by course_id, course_name