Question

How would you model booked hotel room to guests relationship (in PostgreSQL, if it matters)? A room can have several guests, but at least one.

Sure, one can relate guests to bookings with a foreign key booking_id. But how do you enforce on the DBMS level that a room must have at least one guest?

May be it’s just impossible?

Answer 1

Actually, if you read the question, it states booked hotel rooms. This is quite easy to do as follows:

Rooms:     room_id primary key not null     blah     blah  Guests:     guest_id primary key not null     yada     yada  BookedRooms:     room_id primary key foreign key (Rooms:room_id)     primary_guest_id foreign key (Guests:guest_id)  OtherGuestsInRooms:     room_id foreign key (BookedRooms:room_id)     guest_id foreign key (Guests:guest_id) 

That way, you can enforce a booked room having at least one guest while the OtherGuests is a 0-or-more relationship. You can’t create a booked room without a guest and you can’t add other guests without the booked room.

It’s the same sort of logic you follow if you want an n-to-n relationship, which should be normalized to a separate table containing a 1-to-n and an n-to-1 with the two tables.