Like Peter said, you need to check this out on Google. This kind of thing is really common within most applications (higher level drop down then being used to "filter" the lower level drop down)..
E.g.
Dropdown 1: "Select Book Category:"
- Programming
- Design Patterns
- .....
Selecting "Design Patterns" in DropDown 1 Filters DropDown 2 to show:
- Head First Design Patterns
- .. etc etc
Done lots. Most "teach yourself" books will cover similar examples in their data access chapters (most of mine do).
Rob
http://cantgrokwontgrok.blogspot.com