SQL查询帮助

Question

我有这个架构：

Hotel (**hotelNo**, hotelName, city)
Room (**roomNo, hotelNo**, type, price)
Booking (**hotelNo, guestNo, dateFrom**, dateTo, roomNo)
Guest (**guestNo**, guestName, guestAddress)
** denotes primary keys

我必须完成此查询：

• 显示每家酒店及其最常见的房间。

我有这个查询，这不太正确：

SELECT r.hotelno, type, count(*)
FROM Hotel h, room r
WHERE h.hotelNo = r.hotelno
GROUP BY r.hotelNo, type;

这是它输出的内容：

我做错了什么？

Answer 1

如果您正在寻找受欢迎程度，则需要考虑预订表。将Booking表添加到FROM语句，在hotelNo和roomNo上链接，并对Booking表进行计数。这应该给你你想要的计数。

编辑： 以下是一些示例代码（已测试）：

SELECT TOP (100) PERCENT dbo.Hotel.hotelName, dbo.Room.type, COUNT(*) AS Count
FROM dbo.Booking INNER JOIN
dbo.Room ON dbo.Booking.roomNo = dbo.Room.roomNo AND dbo.Booking.hotelNo = dbo.Room.hotelNo 
INNER JOIN dbo.Hotel ON dbo.Room.hotelNo = dbo.Hotel.hotelNo
GROUP BY dbo.Hotel.hotelName, dbo.Room.type
ORDER BY Count DESC

Answer 2

看起来您正在寻找每个酒店特定类型客房预订数量最多的房间类型 - 另一个合计（房间类型预订数量）的总计（最大）。

逐步建立起来。每家酒店每种类型客房的预订数量：

SELECT r.hotelno, r.type, count(*) AS num_bookings
  FROM Booking AS b
  JOIN Room AS r ON b.hotelNo = r.hotelno AND b.roomNo = r.roomNo
 GROUP BY r.hotelNo, r.type;

现在，您需要知道每个酒店的房型最多。这必须分两个阶段完成：

1. 查找酒店任何类型的最大预订数量。
2. 查找具有该最大数量的房间类型。

第一阶段是：

SELECT s.hotelno, MAX(num_bookings) AS max_bookings
  FROM (SELECT r.hotelno, r.type, count(*) AS num_bookings
          FROM Booking AS b
          JOIN Room AS r ON b.hotelNo = r.hotelno AND b.roomNo = r.roomNo
         GROUP BY r.hotelNo, r.type
       ) AS s
 GROUP BY s.hotelno;

第二阶段使用以前的结果作为最终答案：

SELECT t.hotelno, t.type
  FROM (SELECT r.hotelno, r.type, count(*) AS num_bookings
          FROM Booking AS b
          JOIN Room AS r ON b.hotelNo = r.hotelno AND b.roomNo = r.roomNo
         GROUP BY r.hotelNo, r.type) AS t
  JOIN (SELECT s.hotelno, MAX(num_bookings) AS max_bookings
          FROM (SELECT r.hotelno, r.type, count(*) AS num_bookings
                  FROM Booking AS b
                  JOIN Room AS r ON b.hotelNo = r.hotelno AND b.roomNo = r.roomNo
                 GROUP BY r.hotelNo, r.type
               ) AS s
         GROUP BY s.hotelno) AS m
    ON t.hotelno = m.hotelno AND t.num_bookings = m.max_bookings;

如果你的DBMS支持WITH子句，你可以更简洁地写出来。

Answer 3

我认为你将不得不使用内部查询来使这个工作：

SELECT dbo.Hotel.hotelName, pop.type, pop.Count
FROM dbo.Hotel
    INNER JOIN (
        SELECT TOP 1 dbo.Hotel.hotelNo, dbo.Room.type, COUNT(*) AS Count
        FROM dbo.Hotel
            INNER JOIN dbo.Room ON dbo.Room.hotelNo = dbo.Hotel.hotelNo
            INNER JOIN dbo.Booking ON dbo.Booking.roomNo = dbo.Room.roomNo AND dbo.Booking.hotelNo = dbo.Hotel.hotelNo
        GROUP BY dbo.Hotel.hotelNo, dbo.Room.type
        ORDER BY Count DESC, dbo.Room.type
    ) AS pop ON pop.hotelNo = dbo.Hotel.HotelNo
ORDER BY dbo.Hotel.hotelName