Menu item-subitem population from Database

 
Hello
 I am trying to populate menu from database in asp.net 3.5 but as i am new to this technology i really don't know what is the exact way. I tried to find out on web but i found nothing useful for me.

I am developing site for bookstore.
First i want to populate departments and then in department, their categories as subitem.

I have both the tables in database.

I tried following code but it is giving me an erroe on relation code line

This constraint cannot be enabled as not all values have corresponding parent values.

All departments dont have categories.

code :

DataSet DepartmentData = new DataSet();
        DataTable Department = CatalogAccess.GetDepartments();
        DataTable Categories = CatalogAccess.GetCategories();
  &nbs ...