题解 | #计算用户的平均次日留存率#
计算用户的平均次日留存率
http://www.nowcoder.com/practice/126083961ae0415fbde061d7ebbde453
两表进行左连接：
得到结果：
计算t2不为空条数/t1条数：
select count(t2.device_id) / count(t1.device_id) avg_ret
from
(select distinct device_id, date
from question_practice_detail) as t1
left join
(select distinct device_id, date
from question_practice_detail) as t2
on t1.device_id = t2.device_id
and t2.date = date_add(t1.date, interval 1 day)
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