timestampdiff求时间差的函数
语法：timestampdiff(interval(单位),time_start,time_end)

select course_name,

round(avg(timestampdiff(minute,in_datetime,out_datetime)),2) avg_len

from attend_tb a,course_tb b

where a.course_id=b.course_id

group by course_name order by avg_Len desc;