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From: Michael Cole <invalid@invalid.com>
Newsgroups: microsoft.public.sqlserver.programming
Subject: Unique Constraint Based on Dual GUID
Date: Tue, 11 Aug 2015 14:07:19 +1000
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Brief Synopsis: -

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?

For reference, the function would be: -

CREATE FUNCTION GUIDXor ( @guid1 UNIQUEIDENTIFIER, @guid2 
UNIQUEIDENTIFIER)
RETURNS UNIQUEIDENTIFIER
AS
BEGIN

	-- variables
	DECLARE @vb1 BINARY(16), @vb2 BINARY(16), @lo BINARY(8), @hi BINARY(8)

	-- split every 8 bytes into a binary(8), which is a bigint, the 
largest size usable with XOR
	SELECT @vb1 = @guid1, @vb2 = @guid2

	-- xor the high and low parts separately
	SELECT @hi = CONVERT(binary(8), SUBSTRING(@vb1,1,8)) ^ CONVERT(bigint, 
SUBSTRING(@vb2,1,8))
	SELECT @lo = CONVERT(binary(8), SUBSTRING(@vb1,9,8)) ^ CONVERT(bigint, 
SUBSTRING(@vb2,9,8))

	RETURN	CONVERT(UNIQUEIDENTIFIER, @hi + @lo)

END
GO

SELECT dbo.GUIDXor('96B4316D-1EA7-4CA3-8D50-FEE8047C1329', 
'FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF')
SELECT dbo.GUIDXor('96B4316D-1EA7-4CA3-8D50-FEE8047C1329', 
'00000000-0000-0000-0000-000000000000')

-- 
Michael Cole