Question

我对SQL Server比较陌生，所以如果我错过了一些明显的东西，请耐心等待。

我有一个包含表格Retailers，Wholesalers和RetailerWholesaler的数据库。

目前，每个零售商都可以拥有零个，一个或两个批发商（当然，这可能会在不知不觉中发生变化！）。我在创建包含RetailerID，PrimaryWholesalerID和SecondaryWholesalerID列的视图时遇到问题。无论我尝试过什么，我都会在不同的行上继续使用Primary / Secondary：

RetailerID | PrimaryWholesalerID | SecondaryWholesalerID
-----------+---------------------+----------------------
       100 | 8888                | NULL
       100 | NULL                | 9999

我想要达到的目的是：

RetailerID | PrimaryWholesalerID | SecondaryWholesalerID
-----------+---------------------+----------------------
       100 | 8888                | 9999

这是我目前的剧本：

SELECT 
    r.RetailerID,
    CASE WHEN rw.RetailerWholesalerType = 'Primary'
            THEN w.WholesalerID END PrimaryWholesalerID
    CASE WHEN rw.RetailerWholesalerType = 'Secondary'
            THEN w.WholesalerID END SecondaryWholesalerID
FROM
    Retailers r
LEFT OUTER JOIN 
    RetailerWholesaler rw ON r.RetailerID = rw.RetailerID
LEFT OUTER JOIN 
    Wholesalers w ON rw.WholesalerID = w.WholesalerID

即使没有主要或辅助批发商，我仍然希望显示RetailerID。

我正在使用SQL Server 2017.提前感谢您的任何帮助或见解。

Answer 1

您可以使用MAX()

有条件地汇总结果

SELECT r.RetailerID,
       MAX(CASE WHEN rw.RetailerWholesalerType = 'Primary'
                THEN w.WholesalerID END) PrimaryWholesalerID
       MAX(CASE WHEN rw.RetailerWholesalerType = 'Secondary'
                THEN w.WholesalerID END) SecondaryWholesalerID
FROM Retailers r
     LEFT OUTER JOIN RetailerWholesaler rw
         ON r.RetailerID = rw.RetailerID
     LEFT OUTER JOIN Wholesalers w
         ON rw.WholesalerID = w.WholesalerID
GROUP BY r.RetailerID

Answer 2

我建议使用聚合来写这个：

SELECT r.RetailerID,
       MAX(CASE WHEN rw.RetailerWholesalerType = 'Primary'
                THEN w.WholesalerID
           END) as PrimaryWholesalerID
       MAX(CASE WHEN rw.RetailerWholesalerType = 'Secondary'
                THEN w.WholesalerID
           END) as SecondaryWholesalerID
FROM Retailers r LEFT OUTER JOIN
     RetailerWholesaler rw
     ON r.RetailerID = rw.RetailerID LEFT OUTER JOIN
     Wholesalers w
     ON rw.WholesalerID = w.WholesalerID
GROUP BY r.RetailerID;

SQL Server：多对多加入单行

2 个答案: