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(avg(timestampdiff(minute, in_datetime, out_datetime)),2) as avg_Len from attend_tb join course_tb using (course_id) group by course_name order by avg_Len desc