题解 | #计算用户的平均次日留存率#

关键逻辑 同一id的日期+1能够找到匹配值，表的自联结，连续两天上线用户数量/全部上线用户数量 两个易错点： date_add(date,INTERVAL 1day) 容易写城day_add 留存率计算无需过滤重复的id

SELECT COUNT(t2.device_id)/ COUNT(t1.device_id) 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);