题解 | #牛客直播开始时各直播间在线人数#

select c.course_id,course_name,count(distinct user_id) as online_num from course_tb c inner join attend_tb a using(course_id) where date_format(in_datetime,'%H:%i') <= '19:00' and date_format(out_datetime,'%H:%i') > '19:00' group by c.course_id,course_name order by course_id //直播开始时各直播间在线人数可以变成7点前进入直播且7点后退出直播间这两个条件；然后就很简单了