题解 | 牛客直播开始时各直播间在线人数

SELECT c.course_id, c.course_name,
    COUNT(DISTINCT a.user_id) online_num
FROM course_tb c 
JOIN attend_tb a USING(course_id)
WHERE TIME(a.in_datetime) <= '19:00:00' 
    AND TIME(a.out_datetime) > '19:00:00' 
GROUP BY c.course_id, c.course_name
ORDER BY c.course_id