以下SQL查询中的内部查询有什么作用？ min函数中有两个datetime函数是什么意思？

Question

我正在做卡内基梅隆大学https://15445.courses.cs.cmu.edu/fall2018/homework1/上一次数据库系统课程的家庭作业。 我被困在问题4上。作业解决方案中给出的有关SQLite3的正确SQL查询如下，但我对GROUP BY子句之前的ROTFTIME和datetime函数感到困惑，并且从ROUND（SUM ...

开始

WITH dates AS (SELECT date(start_time) AS tdate 
               FROM trip 
               UNION 
               SELECT date(end_time) AS tdate 
               FROM trip) 
SELECT tdate,
   ROUND(SUM(strftime('%s', MIN(datetime(end_time), datetime(tdate, '+1
     day'))) - strftime('%s', MAX(datetime(start_time), datetime(tdate)))) 
     * 1.0 / (SELECT count(distinct(bike_id)) 
              FROM trip 
              WHERE bike_id <= 100), 4) AS avg_duration 
FROM trip, dates 
WHERE bike_id <= 100 
AND datetime(start_time) < datetime(tdate, '+1 day') 
AND datetime(end_time) > datetime(tdate) 
GROUP BY tdate 
ORDER BY avg_duration DESC LIMIT 10;