select round(count(tb2.user_id)/count(tb1.user_id),3)
from(
(select distinct user_id, min(date) as date from login
group by user_id) as tb1
left join 
(select distinct user_id, date as date from login) as tb2
on tb1.user_id=tb2.user_id and datediff(tb2.date,tb1.date)=1
)