Are you using Routing if so make a link like this this Route you use the department ID to select all of the categories in this departement

<asp:HyperLink ID="HyperLink1" runat="server" NavigateUrl ='<%#"/department/"+ Eval("DepartmentNameUrl")+"/"+Eval("DepartmentID") %>'><%# Eval("Name") %></asp:HyperLink>

here is the Route with regex validation

      void RegisterRoutes (System.Web.Routing.RouteCollection routes)       

  {                     

   routes.MapPageRoute("Departments",  

               "department/{name}/{Id}",  

            "~/Department.aspx",true  , 

           new RouteValueDictionary {    { "Id", "[0-9]{1,8}" } });

{

Remember to Register your routes in the application start in the global.ascx file

  void Application_Start(object sender, EventArgs e)         

{            

 RegisterRoutes(System.Web.Routing.RouteTable.Routes);

 }

These tutorials are a good place to start

http://www.asp.net/web-forms/overview/routing