switch( true ) {
    case( CATEGORIES_SORT_ORDER == "products_name" ):
        $order_it_by = "pd.products_name";
        break;

    case( CATEGORIES_SORT_ORDER == "products_name-desc" ):
        $order_it_by = "pd.products_name DESC";
        break;

    case( CATEGORIES_SORT_ORDER == "model" ):
        $order_it_by = "p.products_model";
        break;

    case( CATEGORIES_SORT_ORDER == "model-desc" ):
        $order_it_by = "p.products_model DESC";
        break;

    default:
        $order_it_by = "pd.products_name";
        break;
}