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： 假设您有三个不同的表（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$    |

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

有一点对我来说非常重要：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

以下内容可根据需要使用，

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

您可以使用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

Answer 3

SELECT *
(SELECT Products.ProductID,
        Products.Name,
        'Bill' as Type,
        Bills.Amount
FROM        Products INNER JOIN Bills
            on Products.ProductID = Bills.ProductID
UNION
SELECT  Products.ProductID,
        Products.Name,
        'Ret' as Type,
        Returns.Amount
FROM        Products INNER JOIN Returns
            on Products.ProductID = Returns.ProductID) as iTable
WHERE   iTable.ProductID = 1

Answer 4

尝试UNION查询 - 类似

SELECT *, 'bill' FROM 
dbo.product
INNER JOIN bills ON dbo.product.Product_Id = dbo.bills.product_id
UNION
SELECT *, 'return' FROM 
dbo.product
INNER JOIN dbo.returns ON dbo.product.Product_Id = dbo.returns.product_id

Answer 5

如果您不喜欢嵌套子查询的外观，可以使用GarethD's great solution并将其转换为公用表表达式（CTE）

WITH Transactions AS (
    SELECT ProductID, 'Bill' [Type], Amount FROM Bills
      UNION ALL
    SELECT ProductID, 'Ret'  [Type], Amount FROM Returns
)
SELECT  p.*,  [Type], Amount
FROM    Products p
JOIN    Transactions t
            ON t.ProductID = p.ProductID

Here's a demo in SqlFiddle