使用子查询

select order_num from orderitems where prod_id = 'tnt2';

select cust_id from orders where order_num in ('20005','20007');

等价于

select cust_id from orders where order_num in ( select order_num from orderitems where prod_id = 'tnt2');

select cust_name,cust_contact from customers where cust_id in

    -> (select cust_id from orders where order_num in (

    ->  select order_num from orderitems where prod_id = 'tnt2'));