select
    d.id,
    c.name,
    space(8000) as catlist
into
    #temp
from
    document d
    inner join documentcategory dc
        on d.id = dc.documentid
    inner join category c
        on dc.categoryid = c.id
order by
    d.id desc

declare @lastid int, @catlist varchar(20)
select @lastid = -1, @lastcat = ''

update
    #temp
set
    @catlist = catlist = case
                             when id <> @lastid then name
                             else @catlist + ',' + name
                         end,
    @lastid = id

select
    id,
    max(catlist) as catlist
from
    #temp
group by
    id

drop table #temp