sql - 从一对多的关系中得到一对一的结果

Question

我的情况是我有一对多的关系，例如订单和订单详细信息，我需要获取具有单个订单详细信息的订单。

Answer 1

SELECT O1.order_number
  FROM Orders AS O1
 WHERE 1 = (
            SELECT COUNT(*)
              FROM OrderDetails AS D1
             WHERE O1.order_number = D1.order_number
           );

Answer 2

怎么样：

select * 
from order
where order_number in
    (select order_number
     from order_details
     group by order_number
     having count(*) = 1)