Group By in CAML query

I have the following CAML query:

		listQuery = "<Query>";
		listQuery += "	<GroupBy collapse='true'>";
		listQuery += "    <FieldRef Name='DepartmentName' />";
		listQuery += "  </GroupBy>";
		listQuery += "  <OrderBy>";
		listQuery += "    <FieldRef Name='DepartmentName' />";
		listQuery += "  </OrderBy>";
		listQuery += "  <ViewFields>";
		listQuery += "    <FieldRef Name='DepartmentID' />";
		listQuery += "    <FieldRef Name='DepartmentName' />";
		listQuery += "  </ViewFields>";
		listQuery += "</Query>";

I'm trying to get a unique list of department names.  However, I am getting multiple instances of each department (in essence one per record in the underlying record).  What is wrong with my GroupBy tag?

Thanks,

- Bruce

- Bruce