Drop Down Boxes

I have a form that will add a device to a database. What I am having a problem with is that I have two drop down boxes. One is a building number and the other one is a room name. Because there are many room names that are the same in many of the buildings and some room names that are not in all of the buildings, I have created a table called buildings and another table called rooms.
I have a drop down menu that displays all the building numbers and what I want to be able to do is when I select the specific building number, another dropdown box appears showing all the room names for that specific building. I also what to be able to keep the information already on the form intact. As an example, a user puts in information on the form such as the machine name, username and then selected a building name, which would then show the rooms in that specific building, maintaining the information of the machine name and username on the form. There are seven items on the form, which the user must include prior to submission of the form.