多表格查询数据实例两种方法，好坏自现

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'