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
差点忘记timestampdiff函数的用法
- TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)unit可以表示年月日时分秒，返回差值。
- DATEDIFF(expr1,expr2)返回两个日期差（天数）
select
course_name,
round(sum(TIMESTAMPDIFF(minute,in_datetime,out_datetime))/count(course_name),2) as avg_Len
from course_tb
join attend_tb
using(course_id)
group by course_name
order by avg_Len desc
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