题解 | 计算用户的平均次日留存率

select
count(t2.date)/count(t1.date) as avg_ret
from (
select
distinct device_id,date
from question_practice_detail )as t1
left join 
(
    select 
    distinct device_id,date_add(date,interval 1 day) as date 
    from question_practice_detail
) as t2
on t1.device_id=t2.device_id and  t1.date=t2.date