题解 | 计算用户的平均次日留存率

select count(b.device_id)/count(a.device_id) avg_ret
from (
    select distinct device_id, date
    from question_practice_detail
) a
left join (
    select distinct device_id, date
    from question_practice_detail
) b
on a.device_id = b.device_id and date_add(a.date, interval 1 day)=b.date
# where b.date is not null