select dt,round(sum(s)/count(distinct uid),1) as avg_viiew_len_sec
from (select date(in_time) as dt ,timestampdiff(second,in_time,out_time) as s ,uid
from tb_user_log
where year(in_time) = 2021 and month(in_time) = 11 and artical_id != 0) a
group by dt
order by avg_viiew_len_sec;