多表格查詢資料例項兩種方法，好壞自現

SELECT p.prod_name FROM products p WHERE p.prod_id IN 
(
SELECT DISTINCT o.prod_id FROM orderitems o WHERE o.order_num IN 
   (SELECT   o.order_num FROM customers c RIGHT JOIN  orders o ON c.cust_id = o.cust_id)
);








SELECT p.prod_id,p.prod_name 
FROM products p
LEFT JOIN orderitems oi ON oi.prod_id = p.prod_id
LEFT JOIN orders os ON os.order_num = oi.order_num
LEFT JOIN customers cs ON cs.cust_id = os.cust_id
WHERE cs.cust_name = 'xxx'