从一对多关系中获得一对一的结果

Question

我的情况是，我有一对多的关系，比如订单和订单细节，我需要获得具有单一订单细节的订单。

Answer 1

如何：

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

Answer 2

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