Show all product names without an order.
SELECT product_name FROM Products EXCEPT SELECT T1.product_name FROM Products AS T1 JOIN Order_items AS T2 ON T1.product_id  =  T2.product_id
product names without an order refers to selecting products that do not appear in any order; EXCEPT is used to subtract one set of results from another; JOIN is used to combine rows from two or more tables based on a related column between them.