order by可以用在视图中，如果从该视图检索数据select中也含有order by 那么该视图中的order by将被覆盖
视图不能索引，
视图可以和表一起使用
创建视图
create view productcustomers as select cust_name,cust_contact,prod_id from customers,orders,orderitems where customers.cust_id = orders.cust_id and orderitems.order_num = orders.order_num;
select cust_name,cust_contact from productcustomers where prop_id=’TNT2’;