Show all product names and the number of customers having an order on each product.
SELECT T2.product_name ,  count ( * )  FROM Order_items AS T1 JOIN Products AS T2 ON T1.product_id   =   T2.product_id JOIN Orders AS T3 ON T3.order_id   =   T1.order_id GROUP BY T2.product_name
product names refer to product_name; number of customers having an order on each product refers to COUNT(*) after JOINing Order_items, Products, and Orders tables on product_id and order_id;