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(minute,in_datetime,out_datetime))/count(user_id),2) as avg_Len from attend_tb join course_tb using (course_id) group by course_name order by avg_Len desc;
在时间上的相减，timestampdiff（minute, in_datetime,out_datetime ）
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