Simple CHECK Constraint not so simple

Question

2nd Edit: The source code for the involved function is as follows:

ALTER FUNCTION [Fileserver].[fn_CheckSingleFileSource] ( @fileId INT )
RETURNS INT
AS 
    BEGIN
        -- Declare the return variable here
        DECLARE @sourceCount INT ;

        -- Add the T-SQL statements to compute the return value here
        SELECT  @sourceCount = COUNT(*)
        FROM    Fileserver.FileUri
        WHERE   FileId = @fileId
                AND FileUriTypeId = Fileserver.fn_Const_SourceFileUriTypeId() ;

        -- Return the result of the function
        RETURN @sourceCount ;

    END

Edit: The example table is a simplification. I need this to work as a Scaler Function / CHECK CONSTRAINT operation. The real-world arrangement is not so simple.

Original Question: Assume the following table named FileUri

FileUriId, FileId, FileTypeId

I need to write a check constraint such that FileId are unique for a FileTypeId of 1. You could insert the same FileId as much as you want, but only a single row where FileTypeId is 1.

The approach that DIDN'T work:

1) dbo.fn_CheckFileTypeId returns INT with following logic: SELECT Count(FileId) FROM FileUri WHERE FileTypeId = 1

2) ALTER TABLE FileUri ADD CONSTRAINT CK_FileUri_FileTypeId CHECK dbo.fn_CheckFileTypeId(FileId) <= 1

When I insert FileId 1, FileTypeId 1 twice, the second insert is allowed.

Thanks SO!

Answer 1

You need to create a filtered unique index (SQL Server 2008)

CREATE UNIQUE NONCLUSTERED  INDEX ix ON YourTable(FileId) WHERE FileTypeId=1

or simulate this with an indexed view (2000 and 2005)

CREATE VIEW dbo.UniqueConstraintView
WITH SCHEMABINDING
AS
SELECT FileId
FROM dbo.YourTable
WHERE FileTypeId = 1

GO 
CREATE UNIQUE CLUSTERED  INDEX ix ON dbo.UniqueConstraintView(FileId)

Answer 2

Why don't you make FieldTypeID and Field both the primary key of the table?

Or at least a Unique Index on the table. That should solve your problem.