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Newsgroups: microsoft.public.sqlserver.programming
Date: Tue, 11 Aug 2015 12:46:44 -0700 (PDT)
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Subject: Re: Unique Constraint Based on Dual GUID
From: rpresser <rpresser@gmail.com>
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Xref: csiph.com microsoft.public.sqlserver.programming:31269

On Tuesday, August 11, 2015 at 12:08:02 AM UTC-4, Michael Cole wrote:
> Putting a constrain on these two fields will limit it to only one 
> combination of the two fields, i.e., a link of A to B, but I also need 
> to ensure that the link is not duplicated as B to A - the link is 
> non-directional.
> 
> My idea was to include a calculated field of the two GUIDs XORed 
> together, and place a constraint on this calculated field.  Can anyone 
> see any issues with this idea?

Rather than an XOR, which as Erland points out could cause false positives, why not use a calculated field that appends the two GUIDs together with the one that is "less" coming first?

CREATE FUNCTION OneWayOnly
    (
      @guid1 UNIQUEIDENTIFIER ,
      @guid2 UNIQUEIDENTIFIER
    )
RETURNS VARCHAR(72)
AS
    BEGIN
        DECLARE @g1 VARCHAR(36) ,
            @g2 VARCHAR(36) ,
            @result VARCHAR(72);
        SET @g1 = CONVERT(VARCHAR(36), @guid1);
        SET @g2 = CONVERT(VARCHAR(36), @guid2);
        IF @g1 < @g2
            SET @result = @g1 + @g2;
        ELSE
            SET @result = @g2 + @g1;
        RETURN @result;
    END;