select date_format(in_time,'%Y-%m-%d') as dt, 
	round(sum(timestampdiff(second, in_time, out_time)) / count(distinct uid), 1)
	as 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 2

第一行可以之间用date函数调用，第五行如果没有增加and后面的条件的话会出现错误