Hi Michael,
You could add an OnAfterChange event handler (code) that gets called when they select the department. Or, you could add a rule that fires when a value changes and the rule could query the database based on the value. This is tricky and will require some hardcoding because the query strings don't support passing parameters.
The other possibility is to use our Database Accelerator Suite which will allow you to do this without writing any code.
Good luck!
Patrick Halstead
Project Manager at Qdabra