题解 | 牛客直播开始时各直播间在线人数
with a as (select user_id,course_id,in_datetime dt from attend_tb where TIME(in_datetime) <= '19:00:00') select a.course_id,b.course_name,count(1) from a left join course_tb b on a.course_id=b.course_id group by a.course_id,b.course_name
