Question

我最近注意到在使用LEFT JOIN时，如果您忘记使用WHERE子句，则不会收到任何错误消息，让您构建并运行它。但是WHERE子句之后的代码也不起作用。

检查此样本：

SELECT 
     [C].Id AS Id
    ,[C].Code AS Code
    ,[C].[Desc] AS [Desc]
    ,[C].DateCreated 
    ,[C].UserCreatedId, [U].Username AS UserCreatedUsername, '(' + [U].Username + ') ' + [U].Firstname + ' ' + [U].Lastname AS UserCreatedFirstnameLastname 

    FROM rel_MerchantCurrency [MC]
    LEFT JOIN Merchant [M] ON [MC].MerchantId = [M].Id
    LEFT JOIN Currency [C] ON [MC].CurrencyId = [C].Id
    LEFT JOIN [User] [U] ON [MC].UserCreatedId = [U].Id

    AND [M].Id = @MerchantId

    AND [MC].Deleted = 0
    AND [M].Deleted = 0
    AND [C].Deleted = 0
    AND [U].Deleted = 0

可以毫无问题地构建此代码。但是，它从不检查最后几行。所以它不能正常工作。但是这段代码如下：

 SELECT 
         [C].Id AS Id
        ,[C].Code AS Code
        ,[C].[Desc] AS [Desc]
        ,[C].DateCreated 
        ,[C].UserCreatedId, [U].Username AS UserCreatedUsername, '(' + [U].Username + ') ' + [U].Firstname + ' ' + [U].Lastname AS UserCreatedFirstnameLastname 

        FROM rel_MerchantCurrency [MC]
        LEFT JOIN Merchant [M] ON [MC].MerchantId = [M].Id
        LEFT JOIN Currency [C] ON [MC].CurrencyId = [C].Id
        LEFT JOIN [User] [U] ON [MC].UserCreatedId = [U].Id

        WHERE [M].Id = @MerchantId

        AND [MC].Deleted = 0
        AND [M].Deleted = 0
        AND [C].Deleted = 0
        AND [U].Deleted = 0

这很有效，因为我正在添加“WHERE”子句。

我的问题是：为什么我没有收到第一个代码的任何错误消息？我相信因为代码运行底部的所有AND子句与最后一个LEFT JOIN行。但是它不应该工作吗？你会建议避免这种情况，因为没有抛出异常？我最近更改了很多LEFT JOIN的存储过程，我认为大多数都存在这个问题，我不想再将它再次体验过。

Answer 1

在您的第一个查询中，它被解释为

LEFT JOIN [User] [U] 
   ON (
           [MC].UserCreatedId = [U].Id
       AND [M].Id = @MerchantId
       AND ...
      )

（注意括号）
这就是为什么忘记WHERE时语法错误的原因。您只需限制连接而不是最终结果。

Answer 2

你的第一段代码在语法上是正确的，所以你不会得到错误，但它没有意义。由于您尚未使用WHERE子句，因此最后的所有ANDs都将应用于表[U]和[MC]之间的LEFT JOIN。

您可能不知道，但是您可以将多个条件放入连接中，因此指定类似

的内容非常有效

LEFT JOIN [User] [U] ON [MC].UserCreatedId = [U].Id AND [MC].DateCreated = [U].DateCreated

而不是

的等效陈述

LEFT JOIN [User] [U] ON [MC].UserCreatedId = [U].Id 
WHERE [MC].DateCreated = [U].DateCreated

所以你的SQL基本上是指定

LEFT JOIN [User] [U] ON [MC].UserCreatedId = [U].Id
AND [M].Id = @MerchantId
AND [MC].Deleted = 0
AND [M].Deleted = 0
AND [C].Deleted = 0
AND [U].Deleted = 0

由于您正在对与表[M]和[C]相关的连接应用条件，因此它们将被忽略，因为它们不会影响表[U]和[MC]之间的连接。并且因为它们被视为[U]和[MC]之间连接的条件，所以它们不适用于您想要的连接。

如果您愿意，可以将与已加入表相关的ANDs移动到联接中。这会使您的代码看起来像

SELECT 
     [C].Id AS Id
    ,[C].Code AS Code
    ,[C].[Desc] AS [Desc]
    ,[C].DateCreated 
    ,[C].UserCreatedId, [U].Username AS UserCreatedUsername, '(' + [U].Username + ') ' + [U].Firstname + ' ' + [U].Lastname AS UserCreatedFirstnameLastname 

    FROM rel_MerchantCurrency [MC]
    LEFT JOIN Merchant [M] ON [MC].MerchantId = [M].Id AND [M].Deleted = 0 AND [M].Id = @MerchantId
    LEFT JOIN Currency [C] ON [MC].CurrencyId = [C].Id AND [C].Deleted = 0
    LEFT JOIN [User] [U] ON [MC].UserCreatedId = [U].Id AND [U].Deleted = 0
    WHERE [MC].Deleted = 0

为什么不在LEFT Join之后使用WHERE子句不会抛出任何异常？

2 个答案: