sql - T-SQL : Single-Query Join with either one Table or another

Question

我的问题如下:假设您有三个不同的表(产品、账单和返回)

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

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

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

单个查询如何获得以下输出:

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

我尝试过各种连接，但不知何故我无法理解这个问题。我究竟做错了什么？到目前为止我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，我不想复制/粘贴查询的整个逻辑，就像使用联合时的情况一样。

Answer 1

以下内容将根据需要工作，

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