题解|DATE_FORMAT小时分钟 牛客直播开始时在线人数

SELECT a.course_id,course_name,COUNT(DISTINCT user_id) AS online_num
FROM attend_tb a
LEFT JOIN course_tb b
ON a.course_id = b.course_id
WHERE '19:00' BETWEEN DATE_FORMAT(in_datetime,'%H:%i') AND DATE_FORMAT(out_datetime,'%H:%i')
GROUP BY course_id,course_name
ORDER BY course_id

## 注意DATE_FORMAT 小时分钟