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
https://www.nowcoder.com/practice/e9e7dc4c8623467793f6999cbfee9360
select course_name,round(sum(timestampdiff(second,in_datetime,out_datetime))/60/count(1),2) avg_Len from course_tb join attend_tb using(course_id) group by course_name order by avg_Len desc