题解 | #牛客直播各科目平均观看时长#

select
  cb.course_name,
  round(
    sum(
      timestampdiff(MINUTE, ab.in_datetime, ab.out_datetime)
    ) / count(1),
    2
  ) avg_Len
from
  attend_tb ab
  inner join course_tb cb on ab.course_id = cb.course_id
group by
  cb.course_name
order by
  avg_Len desc