Poněkud zakulacená metoda!
declare @t table
(
i uniqueidentifier default newsequentialid(),
m as cast(i as char(36))
)
insert into @t default values;
select
substring(m,25,2) + '-' +
substring(m,27,2) + '-' +
substring(m,29,2) + '-' +
substring(m,31,2) + '-' +
substring(m,33,2) + '-' +
substring(m,35,2) AS MacAddress
FROM @t