题解 | 计算用户的平均次日留存率
select count(q2.device_id)/count(q1.device_id) as 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 q2.date=date_add(q1.date,interval 1 day)
