select 
    group_concat(distinct DATE_FORMAT(in_time, '%Y-%m-%d')) dt,
	round(sum(TIMESTAMPDIFF(second, in_time, out_time))/ count(distinct uid), 1) avg_viiew_len_sec
from 
	tb_user_log
where
		year(in_time) = 2021
	and 
		month(in_time) = 11
    and
        artical_id != 0
group by 
	day(in_time)
order by
	avg_viiew_len_sec