sql - SQL Join Query - 首先选择特定条目，否则选择 NULL

Question

我敢肯定这个问题之前已经回答过，但我找不到！

表 - 预订 B

Customer    Product
Tim         Milk
Bob         Milk

表 - 产品 P

Customer    Product     Description
Tim         Milk         This is Tim's Milk
NULL        Milk         This is Anybody's Milk

我想加入表格并获得以下输出（为简单起见，显示示例中的所有列）：

B.Customer    B.Product   P.Customer     P.Product    P.Description
Tim           MILK        Tim            MILK         This is Tim's Milk
Bob           MILK        NULL           MILK         This is Anybody's Milk

所以，查询应该首先查找是否有特定的客户相关产品，如果有，使用它，否则使用通用产品......

非常感激！

Answer 1

   SELECT B.Customer, B.Product,
          ISNULL(C.Customer,P.Customer) Customer,
          CASE WHEN C.Customer IS NULL THEN P.Description
               ELSE C.Description END Description
     FROM B
LEFT JOIN C ON C.Product = B.Product and C.Customer = B.Customer
LEFT JOIN P ON P.Product = B.Product and P.Customer IS NULL

Answer 2

我想我们需要加入两次：首先是客户和产品，然后是产品

SELECT
  B.Customer,
  B.Product,
  IFNULL(Pfound.Customer,PnotFound.Customer) AS P_Customer,
  IFNULL(Pfound.Product,PnotFound.Product) AS P_Product,
  IFNULL(Pfound.Description,PnotFound.Description) AS P_Description
FROM B
  LEFT JOIN P AS Pfound
    ON B.Customer=Pfound.Customer
    AND B.Product=Pfound.Product
  LEFT JOIN P AS PnotFound
    ON B.Product=PnotFound.Product
    AND PnotFound.Customer IS NULL