デカルト積とfrom句で選択

デカルト積の理由は、ordersで副選択に参加しなかったためです。 またはPart テーブル。

まず第一に、そのsub-selectは必要ありません

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;