Postgresql-forespørgsel til at få n-niveau overordnet-barn-relation gemt i en enkelt tabel

Med Postgres kan du bruge et rekursivt almindeligt tabeludtryk:

with recursive rel_tree as (
   select rel_id, rel_name, rel_parent, 1 as level, array[rel_id] as path_info
   from relations 
   where rel_parent is null
   union all
   select c.rel_id, rpad(' ', p.level * 2) || c.rel_name, c.rel_parent, p.level + 1, p.path_info||c.rel_id
   from relations c
     join rel_tree p on c.rel_parent = p.rel_id
)
select rel_id, rel_name
from rel_tree
order by path_info;

SQLFiddle baseret på dit eksempel:http://sqlfiddle.com/#!11/59319/19

(Jeg erstattede mellemrummene for indrykning med understregninger, da SQLFiddle ikke viser mellemrummene korrekt)