SQL query assistance - sql

Question

I have this schema:
Hotel (**hotelNo**, hotelName, city)
Room (**roomNo, hotelNo**, type, price)
Booking (**hotelNo, guestNo, dateFrom**, dateTo, roomNo)
Guest (**guestNo**, guestName, guestAddress)
** denotes primary keys
I have to complete this query:
Display each hotel and its most common room.
I have this query, which isn't quite correct:
SELECT r.hotelno, type, count(*)
FROM Hotel h, room r
WHERE h.hotelNo = r.hotelno
GROUP BY r.hotelNo, type;
This is what it outputs:
What am I doing wrong?

Answer 1

It looks as though you're seeking the type of room which has maximum number of bookings for rooms of a given type at each hotel - an aggregate (maximum) of another aggregate (count of bookings of room type).
Build it up piece-wise. The number of bookings of rooms of each type at each hotel:
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;
Now, you need to know which room type has the maximum at each hotel. That has to be done in two stages:
Find the maximum number of bookings at the hotel for any type.
Find the room types with that maximum number.
The first stage is:
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;
The second stage uses both the previous results for a final answer:
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;
If your DBMS supports WITH clauses, you can write that more succinctly.

Answer 2

If you are looking for popularity, you would need to take into account the Booking table. Add the Booking table to your FROM statement, link on hotelNo and roomNo and do a count on the Booking table. This should give you the counts you want.
Edit:
Here is some sample code for you (tested):
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 3

I think you're going to have to use an inner query to get this one working:
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