with a as(
    select 
        ct.course_name,
        round(avg(timestampdiff(second,at.in_datetime,at.out_datetime)/60),2) as avg_Len
    from 
        attend_tb as at
        left join course_tb as ct on at.course_id=ct.course_id
    group by 
        ct.course_name
    order by 
        avg_Len desc
)
select * from a