题解 | #查看不同年龄段的用户明细#题意不清，给的例子也不全
计算用户的平均次日留存率
http://www.nowcoder.com/practice/126083961ae0415fbde061d7ebbde453
SELECT count(distinct q2.device_id,q2.date)/count(distinct q1.device_id,q1.date) as avg_ret
FROM question_practice_detail as q1 left outer join question_practice_detail as q2
on q1.device_id=q2.device_id and datediff(q2.date,q1.date)=1;