Database: customers_and_invoices
Question: Show all product names and the number of customers having an order on each product.
SQL: 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