How to avoid or replace a sub query from Exists statement.

Question

Hello all,
I have a query regarding sql query.
DB 1
Table A                                 Table B
ID   Code   HeaderID            ID   
1    100      1                          1
2    200      1                          2
3    100      2                          3
DB 2
Table C
ID    Code  Type
1     100     1
2     101     1
3     100     2
set @HeaderID = 1
IF EXISTS (SELECT Code From Table A WHERE HeaderID=@ID AND Code  not in (SELECT Code FROM Table C WHERE Type=1))
BEGIN
RAISERROR('code is not set.',16,1)
END
Table A and Table C are in different databases say DB1 and DB2.
How can we replace this with another query using join or something. Can we do it using one query by avoiding sub query.I want to get Code 200 which is not match with Table C Code.
Regards, Ranjith T Rajan Acty System India Pvt Ltd.

Answer 1

Instead of checking later and raising an error you can create a check constraint on the table so that it raises error when you insert the non existent code itself.
You can create a UDF like below
CREATE FUNCTION CheckCodeExistence
@Code int,
@HeaderID int
RETURNS bit
AS
BEGIN
DECLARE @Ret bit
SELECT @Ret = CASE WHEN NOT EXISTS (SELECT 1
FROM DB2.dbo.TableC
WHERE Code = @code
AND [TYPE] = @headerID)
THEN 0
ELSE 1 END
RETURN(@ret)
END
Then create a check constraint on Table A based on it as
ALTER TABLE TableA ADD CONSTRAINT Chk_TableA CHECK (dbo.CheckCodeExistence(Code,HeaderID) =1 )
Now when you try to insert record with Code 200 you'll get the below
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the CHECK constraint "Chk_TableA". The conflict occurred in database "DB1", table "dbo.TableA".
The statement has been terminated.
See similar example here
http://visakhm.blogspot.in/2012/05/implementing-multiple-table-based-check.html
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