With CTE as(
select a.*,
       max(b.num) parent_id
  from accounts a
  left join accounts b on b.num<a.num and b.Hierarchy_level<a.Hierarchy_level
  group by a.num,a.Account_code,a.Account_alias,a.Hierarchy_level
)
select c.*,(
    SELECT Chars = STUFF((
        SELECT ', ' + cast(d.num as varchar)
          FROM CTE d where c.num=d.parent_id
           FOR XML PATH(''), TYPE).value('.', 'VARCHAR(MAX)'), 1, 2, '')
    )
  from CTE c