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 )