1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'order by a.products_sort_order' at line 2
in:
[SELECT a.products_id, b.products_id, b.products_name FROM zen_products a, zen_products_description b where a.products_id = b.products_id AND a.master_categories_id = order by a.products_sort_order]