第一步找到所有id并去重，因为是找新户

select count(distinct user_id) from login

第二步找到自连接产生第二日登陆的id， 并去重

select distinct a.user_id
from login a
join login b
on a.user_id=b.user_id
and b.date = date_add(a.date,INTERVAL 1 DAY)

第三步合并找出比例
select round(count(distinct a.user_id)/(select count(distinct user_id) from login), 3) p
from login a
join login b
on a.user_id=b.user_id
and b.date = date_add(a.date,INTERVAL 1 DAY);