题解 | 计算用户的平均次日留存率 次日留存率

select count(b.device_id)/count(1) as 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 date_add(a.`date`,interval 1 day) = b.`date`
                                                 and a.device_id = b.device_id;