SQL Server: in-query error catching - sql-server

Question

In my query I have to join tables from db that is not under my control. It is driving me mad as sometimes this db is not accessible (please don't ask me why) and this breaks my query. Fields I'm joining are not fundamental for my operations and I want my app to work normally even if these fields are not accessible at a time.
Here's the data structure that I do not own:
[DBOutOfControl].[dbo].[Table1]:
[Field1]
[Field2]
[DBOutOfControl].[dbo].[Table2]:
[Field1]
[Field2]
[Field3]
And here is my table:
[DBInMyControl].[dbo].[Table3]:
[Field1]
My original query looks something like that:
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]
How can i bullet-proof this query so when [ForeignDataQry] generates an error the result would be:
[MyID] [A] [B]
1 NULL NULL
Otherwise
[MyID] [A] [B]
1 Va1 Val2
Is there something that could be done server side?

Answer 1

Just specify the expected result of COUNT, the three names, and you can check tables beforehand. A minor rewrite can allow this to check for objects other than tables, utilize EXISTS if desired, skip or add more checks, etc.:
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

I'd wrap the problematic query in dynamic code in order to be able to catch the compilation error (that we cannot catch in the same scope) like this:
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
Here I use cast(null as ... )as [A] to get the same type as [ForeignDataQry].[A] has, for example if [ForeignDataQry].[A] is int there should be int: cast(null as int )as [A]

Answer 3

I dealt with this problem in a different way that I originally wanted but anyway:
I created a [Table4] that keeps copy of the records from foreign tables - fields matches [ForeignDataQry] + timestamp. I created procedure:
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
I will call this every time when I start my app and handle error there and modify my main LEFT JOIN to refer [Table4]