This doesn't answer the question directly, but hopefully it helps you come up with a solution.

with rCTE as (

select

ag.id,

ag.catlevel,

ag.slug_nl,

CatString = cast(ag.slug_nl as nvarchar(max)),

SortKey = cast(ag.id as varbinary(max))

from

[dbo].[articlegroups] ag

where

ag.parentid = 0

union all

select

ag.id,

ag.catlevel,

ag.slug_nl,

CatString = r.CatString + N' > ' + ag.slug_nl,

SortKey = r.SortKey + cast(ag.id as varbinary(max))

from

rCTE r

inner join [dbo].[articlegroups] ag

on (r.id = ag.parentid)

)

select * from rCTE order by SortKey;