SELECT c.cust_name, o.order_num, ( SELECT SUM(item_price*quantity) FROM OrderItems os WHERE os.order_num = o.order_num ) AS OrderTotal FROM Customers c LEFT JOIN Orders o ON o.cust_id = c.cust_id ORDER BY c.cust_name, o.order_num;

方法一: select c.cust_name, o.order_num from Customers c inner join Orders o on c.cust_id = o.cust_id order by c.cust_name,o.order_num; 方法二: select c.cust_name, o.order_num from Customers c left join Orders o on c.cust_id = o.cust_id order by c.cust_name,o.order_num;