SQL Server：查询中的错误捕获

Question

在我的查询中，我必须从不受我控制的 db 中加入表。这让我发疯，因为有时无法访问此数据库（请不要问我为什么），这会破坏我的查询。我要加入的字段对我的运营来说不是基本的，我希望我的应用能够正常工作，即使这些字段一次无法访问。

这是我不拥有的数据结构：

[DBOutOfControl].[dbo].[Table1]:
[Field1]
[Field2]

[DBOutOfControl].[dbo].[Table2]:
[Field1]
[Field2]
[Field3]

这是我的桌子：

[DBInMyControl].[dbo].[Table3]:
[Field1]

我的原始查询看起来像这样：

SELECT [Table3].[MyID],
       [ForeignDataQry].[A],
       [ForeignDataQry].[B]
FROM [DBInMyControl].[dbo].[Table3]
LEFT JOIN
  (SELECT [Table1].[Field1] AS [MyID],
          [Table1].[Field2] AS [A],
          [SubQry].[Field2] AS [B]
   FROM [DBOutOfControl].[dbo].[Table1]
   LEFT JOIN
     (SELECT [Table2].[Field1],
             [Table2].[Field2]
      FROM [DBOutOfControl].[dbo].[Table2]
      WHERE [Table2].[Field3] = 'Where') AS [SubQry] ON [Table1].[Field1] = [SubQry].[Field1]) AS [ForeignDataQry] ON [Table3].[MyID]=[ForeignDataQry].[MyID]

我怎样才能对这个查询进行防弹，所以当 [ForeignDataQry] 生成错误时，结果将是：

[MyID]  [A]  [B]
1       NULL NULL

否则

[MyID]  [A]  [B]
1       Va1  Val2

有什么可以在服务器端做的吗？

Answer 1

只要指定COUNT的预期结果，三个名字，就可以事先查表。轻微的重写可以允​​许它检查表以外的对象，如果需要，使用EXISTS，跳过或添加更多检查等：

IF 0 = ( -- Specify how many records you expect to come.
    SELECT COUNT(C.[name]) AS [COUNT]
    FROM sys.objects AS O
    LEFT JOIN sys.schemas AS S ON S.schema_id = O.schema_id
    LEFT JOIN sys.columns AS C ON C.object_id = O.object_id
    WHERE O.[name] = 'tablename'
      AND S.[name] = 'schemaname'
      AND C.[name] = 'columnname'
)
    SELECT 1 AS A -- Do some code.
ELSE
    SELECT 2 AS B -- Do some other code.

Answer 2

我会将有问题的查询包装在动态代码中，以便能够像这样捕获编译错误（我们无法在同一范围内捕获）：

    begin try
    declare @sql varchar(4000) =

        'SELECT [Table3].[MyID],
               [ForeignDataQry].[A],
               [ForeignDataQry].[B]
        FROM [DBInMyControl].[dbo].[Table3]
        LEFT JOIN
          (SELECT [Table1].[Field1] AS [MyID],
                  [Table1].[Field2] AS [A],
                  [SubQry].[Field2] AS [B]
           FROM [DBOutOfControl].[dbo].[Table1]
           LEFT JOIN
             (SELECT [Table2].[Field1],
                     [Table2].[Field2]
              FROM [DBOutOfControl].[dbo].[Table2]
              WHERE [Table2].[Field3] = ''Where'') AS [SubQry] ON [Table1].[Field1] = [SubQry].[Field1]) AS [ForeignDataQry] ON [Table3].[MyID]=[ForeignDataQry].[MyID]'
       exec(@sql)
    end try
    begin catch
        SELECT [Table3].[MyID],
               cast(null as ... )as [A],
               cast(null as ...) as [B]
        FROM [DBInMyControl].[dbo].[Table3]
    end catch

这里我使用cast(null as ... )as [A] 来获得与[ForeignDataQry].[A] 相同的类型，例如如果[ForeignDataQry].[A] 是int，则应该是int：cast(null as int )as [A]

Answer 3

我以与我最初想要的不同的方式处理这个问题，但无论如何：

我创建了一个 [Table4] 来保存外部表中的记录副本 - 字段匹配 [ForeignDataQry] + 时间戳。我创建了程序：

CREATE PROCEDURE [dbo].[CopyForeignData] 
AS

DECLARE @Timestamp datetime
SET @Timestamp = getdate()

BEGIN
    INSERT INTO [DBInMyControl].[dbo].[Table4] ([MyID], [A], [B], [Timestamp])
    SELECT [Table1].[Field1] AS [MyID],
          [Table1].[Field2] AS [A],
          [SubQry].[Field2] AS [B],
          @Timestamp
   FROM [DBOutOfControl].[dbo].[Table1]
   LEFT JOIN
     (SELECT [Table2].[Field1],
             [Table2].[Field2]
      FROM [DBOutOfControl].[dbo].[Table2]
      WHERE [Table2].[Field3] = 'Where') AS [SubQry] ON [Table1].[Field1] = [SubQry].[Field1]

    DELETE FROM [DBInMyControl].[dbo].[Table4] WHERE [Timestamp] <> @Timestamp
END

每当我启动我的应用程序并在那里处理错误并修改我的主 LEFT JOIN 以引用 [Table4] 时，我都会调用它