Show all product names
select product_name from products
Which of them has an order?
select t1.product_name from products as t1 join order_items as t2 on t1.product_id = t2.product_id
Which of them don't?
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