Question

选择声明，返回位于独特城市和州的每个供应商的名称，城市和州（即，排除与其他供应商具有相同城市和州的供应商）

SELECT 
    VendorName, VendorCity, VendorState
FROM 
    Vendors
WHERE 
    VendorState + VendorCity NOT IN (SELECT VendorState + VendorCity
                                     FROM Vendors
                                     GROUP BY VendorState + VendorCity
                                     HAVING COUNT(*) > 1)
ORDER BY 
    VendorState, VendorCity;

替代回答

SELECT 
    VendorName, VendorCity, VendorState
FROM 
    Vendors AS Vendors_Main
WHERE 
    VendorCity + VendorState NOT IN (SELECT VendorCity + VendorState
                                     FROM Vendors AS Vendors_Sub
                                     WHERE Vendors_Sub.VendorID <> Vendors_Main.VendorID)
ORDER BY 
    VendorState, VendorCity;

我理解第一个答案，但不了解备用查询。混淆点：下面的行不会返回0行，因为它们引用同一个表而没有额外的where子句吗？

WHERE Vendors_Sub.VendorID <> Vendors_Main.VendorID)

Answer 1

SELECT  VendorName ,
        VendorCity ,
        VendorState
FROM    Vendors AS Vendors_Main
WHERE   VendorCity + VendorState NOT IN 
    (
        SELECT  VendorCity + VendorState
        FROM    Vendors AS Vendors_Sub
        WHERE   Vendors_Sub.VendorID <> Vendors_Main.VendorID
    )
ORDER BY VendorState ,VendorCity;

子查询

SELECT  VendorCity + VendorState
FROM    Vendors AS Vendors_Sub
WHERE   Vendors_Sub.VendorID <> Vendors_Main.VendorID

将为主查询中的每一行返回vendorCity + vendorState的所有现有组合，但不包括具有相同ID的行。想象子查询是为主查询中的每一行调用的函数。

如果WHERE Vendors_Sub.VendorID <> Vendors_Main.VendorID不存在，那么每个主查询行都会在子查询中与自身匹配，整个查询将不返回任何行，因为这些组合都不是唯一的。

Answer 2

WHERE Vendors_Sub.VendorID <> Vendors_Main.VendorID)不会比较同一个表的同一行。

对于外部查询中的每一行，相关子查询在逻辑上执行一次，在这种情况下，它会检查具有相同VendorCity / VendorState组合的行，但不同VendorIDs。

事实上，我希望直接翻译成相关的NOT EXISTS：

SELECT VendorName, VendorCity, VendorState
FROM Vendors AS Vendors_Main
WHERE NOT EXISTS
 (
   SELECT *
   FROM Vendors AS Vendors_Sub
   WHERE Vendors_Sub.VendorCity  = Vendors_Main.VendorCity  -- same city
     AND Vendors_Sub.VendorState = Vendors_Main.VendorState -- same state
     AND Vendors_Sub.VendorID   <> Vendors_Main.VendorID    -- different vendor
 )
ORDER BY VendorState, VendorCity;

这样可以防止'state' + 'acity'与'statea' + 'city'之类的误报，这些误报既可以连接到'stateacity'，也适用于任何类型的数据类型。

SQL Server - 相关子查询/自连接

2 个答案: