Describe the relations that would be produced by the following relational algebra operations:

Question

What will be an ideal response?

Answer 1

a) ?hotelNo (?price ? 50 (Room) )
This will produce a relation with a single attribute (hotelNo) giving the number of those
hotels with a room price greater than £50.
b) ?Hotel.hotelNo ? Room.hotelNo(Hotel ? Room)
This will produce a join of the Hotel and Room relations containing all the attributes of
both Hotel and Room (there will be two copies of the hotelNo attribute). Essentially this
will produce a relation containing all rooms at all hotels.
c) ?hotelName (Hotel Hotel.hotelNo ? Room.hotelNo (?price ? 50 (Room)) )
This will produce a join of Hotel and those tuples of Room with a price greater than £50.
Essentially this will produce a relation containing all hotel names with a room price above
£50.
d) Guest (?dateTo ? ‘1-Jan-2007’ (Booking))
This will produce a (left outer) join of Guest and those tuples of Booking with an end date
(dateTo) greater than or equal to 1-Jan-2007. All guests who don’t have a booking with
such a date will still be included in the join. Essentially this will produce a relation
containing all guests and show the details of any bookings they have beyond 1-Jan-2002.
e) Hotel Hotel.hotelNo ? Room.hotelNo (?price ? 50 (Room)) )
This will produce a (semi) join of Hotel and those tuples of Room with a price greater
than £50. Only those Hotel attributes will be listed. Essentially this will produce a relation
containing all the details of all hotels with a room price above £50.
f) ?guestName, hotelNo (Booking Booking.guestNo ? Guest.guestNo Guest) ?
?hotelNo (?city ??’London’(Hotel))