pg sample data

-- drop table bleh;

create table bleh (id char(2) Primary Key, 
parent_id char(2) );

insert into bleh VALUES ('A1', null);
insert into bleh VALUES ('A2', 'A1');
insert into bleh VALUES ('A3', 'A2');
insert into bleh VALUES ('A4', 'A3');
insert into bleh VALUES ('B1', null);
insert into bleh VALUES ('B2', 'B1');
insert into bleh VALUES ('B3', 'B1');
insert into bleh VALUES ('B4', 'B2');


WITH RECURSIVE p(id, parent_id) AS (
	SELECT id, parent_id 
	FROM bleh 
	WHERE bleh.id = 'B2' -- change to get a different tree/entry point
  UNION ALL 
        SELECT b.id, b.parent_id
        FROM p p
        JOIN bleh b
          ON b.id = p.parent_id),
 c(id, parent_id) AS (
       SELECT id, parent_id
       FROM p
       WHERE p.parent_id IS NULL
  UNION ALL
       SELECT b.id, b.parent_id 
       FROM c c 
       JOIN bleh b
       ON b.parent_id = c.id)          
SELECT * FROM c
ORDER BY id;