在 WHERE 子句中使用 CASE

Question

我有 2 個表需要根據幾個參數進行連接。 其中一個參數是年份。 一個表包含當前年份，而另一年份不包含當前年份，因此必須使用最近的年份並與其他參數匹配。

例子

產品

-------------------------------------------------------------------------------
| product_id | category_id | sub_category_id | product_year | amount |
-------------------------------------------------------------------------------
| 504        | I           | U               | 2020         | 400    |
| 510        | I           | U               | 2019         | 100    |
| 528        | I           | U               | 2019         | 150    |
| 540        | I           | U               | 2018         | 1000   |

折扣

-----------------------------------------------------------------------------
| discount_year | category_id | sub_category_id | discount |
-----------------------------------------------------------------------------
| 2018          | I           | U               | 0.15     |
| 2017          | I           | U               | 0.35     |
| 2016          | I           | U               | 0.50     |

輸出

-----------------------------------------------------------------------------
| product_id | category_id | sub_category_id | product_year | discount_year |
-----------------------------------------------------------------------------
| 504        | I           | U               | 2020         | 2018          |
| 510        | I           | U               | 2019         | 2018          |
| 528        | I           | U               | 2019         | 2018          |
| 540        | I           | U               | 2018         | 2017          |

折扣總是從一年后獲得，但如果這些費率不可用，那么它會一直回溯一年直到可用。

我嘗試了以下方法：

SELECT 
    product_year, a.product_id, a.category_id, a.sub_category_id, 
    discount_year, amount, discount
FROM
    Product a
INNER JOIN 
    Discount b ON a.category_id = b.category_id 
               AND a.sub_category_id  = b.sub_category_id
               AND product_ year = CASE
                                      WHEN discount_year + 1 = product_year 
                                         THEN discount_year + 1
                                      WHEN discount_year + 2 = product_year 
                                         THEN discount_year + 2
                                      WHEN discount_year + 3 = product_year 
                                         THEN discount_year + 3
                                   END
 WHERE 
     product = 540

這將返回以下輸出：

--------------------------------------------------------------------------------------------------
| product_year | product_id | category_id | sub_category_id | discount_year | amount | discount |
--------------------------------------------------------------------------------------------------
| 2016         | 540        | I           | U               | 2017          | 1000   | 0.50     |
| 2017         | 540        | I           | U               | 2017          | 1000   | 0.35     |

任何幫助將不勝感激。

Answer 1

您可以使用OUTER APPLY和子查詢。 在子查詢中，使用TOP和ORDER BY選擇具有最大discount_year的行，即小於product_year 。

SELECT p.product_year,
       p.product_id,
       p.category_id,
       p.sub_category_id,
       d.discount_year,
       p.amount,
       d.discount
       FROM product p
            OUTER APPLY (SELECT TOP 1
                                *
                                FROM discount d
                                WHERE d.category_id = p.category_id
                                      AND d.sub_category_id = p.sub_category_id
                                      AND d.discount_year < p.product_year
                                ORDER BY d.discount_year DESC) d;

Answer 2

您可以使用子查詢代替 CASE 表達式來選擇小於您的product_year的 TOP 1 相關discount_year ，ORDER BY discount_year ASC。

Answer 3

首先使用 CTE 創建產品折扣映射。 這包含從產品表中每個產品年份的折扣表中提取的折扣年份和相應的 product_id。 在此之后，您可以輕松地加入相關表以獲得結果並根據需要消除任何空值

簡化的查詢。

 ;WITH disc_prod_mapper 
    AS
    (
    SELECT  product_id, product_year,(SELECT MAX(discount_year) FROM  #Discount b WHERE discount_year < product_year AND a.category_id  = b.category_id  AND a.sub_category_id   = b.sub_category_id ) AS discount_year
     FROM Product a  
    )
    SELECT a.product_year, c.discount_year, a.amount, c.discount
    FROM Product a
      LEFT JOIN disc_prod_mapper b   ON a.product_id  = b.product_id   
      LEFT JOIN Discount c ON b.discount_year = c.discount_year