Question

我的问题类似于： How do you group by one column and retrieve a row with the minimum value of another column in T/SQL?

首先：我使用的是SQL Server Compact，这使得SQL非常严格。

我有两张桌子，他们之间有多对多关系：房间和课程。从房间列表中我想检查房间是否在指定的时间范围内无人居住，这个房间现在有什么课程，这个房间的下一个课程是什么。前两部分没问题：

SELECT a.Id, COUNT(c.Id) AS ConflictingCourses, d.Id AS CurrentCourseId
FROM Rooms AS a 
    LEFT OUTER JOIN RoomCourses AS b ON b.RoomId = a.Id
    -- Conflicting Courses
    LEFT OUTER JOIN Courses AS c ON c.Id = b.CourseId AND @startTime <= c.EndTime AND @endTime >= c.StartTime
    -- Current Course  
    LEFT OUTER JOIN Courses AS d ON d.Id = b.CourseId AND @now <= d.EndTime AND @now >= d.StartTime 
WHERE (a.Id IN (1,2,3,4))
GROUP BY a.RoomId, d.Id

现在问题：我想要获得第一个具有StartTime的课程。为了学习这门课程，我尝试加入所有课程。从这个结果我认为下一个课程很容易，因为对于每个小组，它是具有较小日期的行的ID：

SELECT a.Id, COUNT(c.Id) AS ConflictingCourses, d.Id AS CurrentCourseId
FROM Rooms AS a 
    LEFT OUTER JOIN RoomCourses AS b ON b.RoomId = a.Id
    -- Conflicting Courses
    LEFT OUTER JOIN Courses AS c ON c.Id = b.CourseId AND @startTime <= c.EndTime AND @endTime >= c.StartTime
    -- Current Course  
    LEFT OUTER JOIN Courses AS d ON d.Id = b.CourseId AND @now <= d.EndTime AND @now >= d.StartTime 
    -- All Upcoming Courses
    LEFT OUTER JOIN Courses AS e ON e.Id = b.CourseId AND e.StartTime >= @now
WHERE (a.Id IN (1,2,3,4))
GROUP BY a.RoomId, d.Id

但是我只能使用MIN（e.StartTime）获得下一个课程的startTime。但是我想要MIN（e.StartTime）所属的行的Id。我尝试了数百种组合，变通方法和其他东西，只是为了获得即将上课的ID。主要问题是SQL CE不支持返回标量值的subquerys，因此我无法接收任何允许我只获得下一个课程的信息。

我有什么想法可以获得即将到来的房间课程吗？

Answer 1

您是否尝试过以下操作？

SELECT a.Id, COUNT(c.Id) AS ConflictingCourses, d.Id AS CurrentCourseId
FROM Rooms AS a 
    LEFT OUTER JOIN RoomCourses AS b ON b.RoomId = a.Id
    -- Conflicting Courses
    LEFT OUTER JOIN Courses AS c ON c.Id = b.CourseId AND @startTime <= c.EndTime AND @endTime >= c.StartTime
    -- Current Course  
    LEFT OUTER JOIN Courses AS d ON d.Id = b.CourseId AND @now <= d.EndTime AND @now >= d.StartTime 
    -- All Upcoming Courses
    LEFT OUTER JOIN Courses AS e ON e.Id IN (SELECT MIN(Id) 
                                             FROM Courses
                                             WHERE Id = b.CourseId
                                               AND e.StartTime >= @now)
WHERE (a.Id IN (1,2,3,4))
GROUP BY a.RoomId, d.Id

我知道IN在这里没用，因为查询总是只返回1个结果，但它显然是这种情况的解决方法（SQL CE）。

使用SQL Compact Edition选择组中最小行的ID

1 个答案: