Champion, you can make your life easier and not distinguish if the room is either booked or not. When performing the search, just retrieve all records that match the search performed by the user, no matter if they are booked or not.
The client ID is the customer ID. When booking a room, this ID will be added to the customer field of a record. So, to book a room, you'll just have to update a record with this number. Since your application will not interact with these numbers, then you may assume that any 8-digit number provided in your GUI when booking a room is a valid number.