Cartesiaanse producten en selecteert in de van-clausule

De reden voor het Cartesiaanse product is dat je niet deelnam aan de subselectie met orders of Part tafel.

Allereerst heb je die sub-select niet nodig

SELECT customer.customer_name, 
       orders.order_date, 
       order_line.num_ordered, 
       order_line.quoted_price, 
       part.descript, 
       order_line.num_ordered * part.price AS amt_billed 
FROM   customer 
       JOIN orders 
         ON customer.customer_num = orders.customer_num 
       JOIN order_line 
         ON orders.order_num = order_line.order_num 
       JOIN part 
         ON order_line.part_num = part.part_num;