题解 | #计算用户的平均次日留存率#

--熟悉LEFT JOIN / DATE_ADD() 等语句和函数的运用
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)