CREATE TABLE Order_items (product_id VARCHAR2, order_id VARCHAR2)
CREATE TABLE Products (product_name VARCHAR2, product_id VARCHAR2)
CREATE TABLE Orders (order_id VARCHAR2)

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
SELECT "T2"."product_name", COUNT(*) FROM "Order_items" "T1" JOIN "Products" "T2" ON "T1"."product_id" = "T2"."product_id" JOIN "Orders" "T3" ON "T1"."order_id" = "T3"."order_id" GROUP BY "T2"."product_name"