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 
    #左边的是可能有第二天没答的，右边的是两天都答的 
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)