题解 | #牛客直播各科目平均观看时长#
牛客直播各科目平均观看时长
http://www.nowcoder.com/practice/e9e7dc4c8623467793f6999cbfee9360
select course_name,round(sum(timestampdiff (minute,in_datetime,out_datetime)) /count(user_id),2) from attend_tb x join course_tb y using(course_id) group by 1 order by 2 desc
