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