sql - 1対多の関係から1対1の結果を得る

Question

私の状況では、注文や注文の詳細など、1対多の関係があり、単一の注文の詳細を持つ注文を取得する必要があります。

Answer 1

How about:

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
           );