Sorting on Multiple Fields

Hi,

I am using Dynamics Nav 2009 classic. I need to sort list of records on a form that comes from one table.

Let's say this is an item list. I need to sort it first by "Item Category Code" ascending, and then by "Product Group Code" descending. If I add those two fields as a key, it will take only one sort order, ascending or descending for both fields.

How I do it programmatically, maybe on "OnOpenForm" trigger?

Thank you.