SQL连接查询 - 首先选择特定条目，否则选择NULL

Question

我确定之前已经回答过，但我找不到它！

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

我想加入表并获得以下输出（为简单起见，在示例中显示所有列）：

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

因此，查询应首先查找是否存在特定的客户相关产品，如果是，请使用它，否则使用通用产品......

非常感谢！

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