SELECT date(in_time) dt, 
ROUND(
sum(TIMESTAMPDIFF(second,in_time,out_time))/count(DISTINCT uid),
1
) avg_viiew_len_sec
from tb_user_log
WHERE DATE_FORMAT(in_time,'%Y-%m') = '2021-11' and artical_id<>0
GROUP BY dt
order by avg_viiew_len_sec