Aggregate Function on Uniqueidentifier (GUID)

Question

Let\'s say I have the following table:

category | guid
---------+-----------------------
   A     | 5BC2...
   A     | 6A1C...
   B     | 92A2...

Answer 1

declare @T table(category char(1), guid uniqueidentifier) 

insert into @T 
select 'a', newid() union all
select 'a', newid() union all
select 'b', newid()

select
  S.category,
  S.guid
from
(  
  select
    T.category,
    T.guid,
    row_number() over(partition by T.category order by (select 1)) as rn
  from @T as T
) as S
where S.rn = 1

If you are on SQL Server 2000 you could to this

select 
  T1.category,
  (select top 1 T2.guid 
   from @T as T2
   where T1.category = T2.category) as guid
from @T as T1
group by T1.category