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From: Erland Sommarskog <esquel@sommarskog.se>
Newsgroups: microsoft.public.sqlserver.programming
Subject: Re: Unique Constraint Based on Dual GUID
Date: Tue, 11 Aug 2015 21:35:46 +0200
Organization: Erland Sommarskog
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Michael Cole (invalid@invalid.com) writes:
> I have a table with a GUID as PK (created via NewId())
> 
> I also have a table to indicate links between records in this table, 
> with a dual PK of GUIDS (for the two records that connect to each 
> other)
> 
> 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?
> 

There is obviously a risk for false positives:

SELECT CASE WHEN dbo.GUIDXor('DF468CF1-9035-477D-BE8B-3E597509F8E2',	
                             '1B05A03D-B830-43A8-8396-9FE8364F952C') = 
                 dbo.GUIDXor('DF468CF1-9035-43A8-BE8B-3E597509F8E2',	
                             '1B05A03D-B830-477D-8396-9FE8364F952C')
            THEN 1
            ELSE 0
       END

Here I have simply swapped a number of bytes in the GUID. What the
actual probability for this to happen, I don't know though.

You need to create the functon WITH SCHEMABINDING, but you may already
have discovered that.

-- 
Erland Sommarskog, Stockholm, esquel@sommarskog.se