题解 | 计算用户的平均次日留存率

select
    count(q2.device_id) / count(q1.device_id) avg_ret
from
    (
        select distinct
            device_id, date
        from
            question_practice_detail
    ) as q1
    left join (
        select distinct
            device_id, date
        from
            question_practice_detail
    ) as q2 on q1.device_id = q2.device_id and DATEDIFF(q1.date, q2.date) = 1