T-SQL:使用一个表或另一个表进行单查询连接

Question

我的问题如下:假设你有三个不同的表(产品,票据和退货)

| ProductId | Name |
=====================
| 1         | Car  |

| BillId | ProductId | Amount |
=================================
| 1      | 1         | 100$   |
| 2      | 1         | 200$   |

| ReturnId | ProductId | Amount |
===================================
| 1        | 1         | 50$    |

SINGLE Query如何获得以下输出:

| Product-ID | Name | Type | Amount |
=====================================
| 1          | Car  | Bill | 100$   |
| 1          | Car  | Bill | 200$   |
| 1          | Car  | Ret  | 50$    |

我正在尝试各种各样的联盟,不知何故,我无法理解这一点.我究竟做错了什么？我到目前为止找到的最接近的解决方案是这样的:

SELECT p.*,
       (CASE
           WHEN b.Amount IS NOT NULL THEN 'Bill'
           ELSE 'Ret'
       END) AS Type,
       COALESCE(b.Amount, r.Amount) AS Amount
FROM Products p
LEFT JOIN Bills b ON b.ProductId = p.ProductId
LEFT JOIN Returns r ON r.ProductId = p.ProductId

有一件事对我来说非常重要:真正的场景查询要大得多,而且我不想复制/粘贴查询的整个逻辑,就像使用联盟时的情况一样.

Answer 1

以下将根据需要工作,

SELECT  Products.*, 
        [Type], 
        Amount
FROM    Products
        INNER JOIN
        (   SELECT  ProductID, 'Bill' [Type], Amount
            FROM    Bills
            UNION ALL
            SELECT  ProductID, 'Ret' [Type], Amount
            FROM    Returns
        ) transactions
            ON transactions.ProductID = Products.ProductID

Answer 2

你可以使用UNION:

SELECT p.*, 'Bill' as [Type], b.Amount
FROM Products p
  INNER JOIN Bills b
    ON b.ProductId = p.ProductId
UNION 
SELECT p.*, 'Ret' as [Type], r.Amount
FROM Products p
  INNER JOIN Returns r
    ON r.ProductId = p.ProductId