PostgreSQL example of recursive queries

neverending.psql

INSERT INTO tree (id, parent_id, description)
VALUES
  (6, 6, 'Im Recursive');

WITH RECURSIVE parents as (
   select id,
          description,
          parent_id
   from tree
   where parent_id = 6
   union all
   select child.id,
          child.description,
          child.parent_id
   from tree as child
     join parents on parents.id = child.parent_id
)
select * from parents;

queries.psql

CREATE TABLE tree(
  id SERIAL PRIMARY KEY,
  parent_id INTEGER,
  description VARCHAR
);

INSERT INTO tree (id, parent_id, description)
VALUES
  (1, null, 'level 0'),
  (2, 1, 'level 1'),
  (3, 2, 'level 2'),
  (4, null, 'Another level 0'),
  (5, 4, 'Another level 1');

WITH RECURSIVE parents as (
   select id,
          description,
          parent_id
   from tree
   where parent_id IS NULL
   union all
   select child.id,
          child.description,
          child.parent_id
   from tree as child
     join parents on parents.id = child.parent_id
)
select * from parents;