Db Design of a Room Booking System
How do you know how long a room is booked for?  Your model indicates when a booking starts, but when does it end?

Also, your ERD shows foreign keys from BOOKING to USER and ROOM but these columns aren't acknowledged in your BOOKING table.  Some other answers have interpreted that as the columns being missing.  You should be explicit in your diagram to avoid confusion (or add the FK columns if you missed them!)