從一對多關係中獲得一對一的結果

Question

我的情況是，我有一對多的關係，比如訂單和訂單細節，我需要獲得具有單一訂單細節的訂單。

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 
      );