PostgreSQL recursive parent/child query - postgresql

Question

I'm having some trouble working out the PostgreSQL documentation for recursive queries, and wonder if anyone might be able to offer a suggestion for the following.
Here's the data:
Table "public.subjects"
Column | Type | Collation | Nullable | Default
-------------------+-----------------------------+-----------+----------+--------------------------------------
id | bigint | | not null | nextval('subjects_id_seq'::regclass)
name | character varying | | |
Table "public.subject_associations"
Column | Type | Collation | Nullable | Default
------------+-----------------------------+-----------+----------+--------------------------------------------------
id | bigint | | not null | nextval('subject_associations_id_seq'::regclass)
parent_id | integer | | |
child_id | integer | | |
Here, a "subject" may have many parents and many children. Of course, at the top level a subject has no parents and at the bottom no children. For example:
parent_id | child_id
------------+------------
2 | 3
1 | 4
1 | 3
4 | 8
4 | 5
5 | 6
6 | 7
What I'm looking for is starting with a child_id to get all the ancestors, and with a parent_id, all the descendants. Therefore:
parent_id 1 -> children 3, 4, 5, 6, 7, 8
parent_id 2 -> children 3
child_id 3 -> parents 1, 2
child_id 4 -> parents 1
child_id 7 -> parents 6, 5, 4, 1
Though there seem to be a lot of examples of similar things about I'm having trouble making sense of them, so any suggestions I can try out would be welcome.

Answer 1

To get all children for subject 1, you can use
WITH RECURSIVE c AS (
SELECT 1 AS id
UNION ALL
SELECT sa.child_id
FROM subject_associations AS sa
JOIN c ON c.id = sa. parent_id
)
SELECT id FROM c;

Answer 2

CREATE OR REPLACE FUNCTION func_finddescendants(start_id integer)
RETURNS SETOF subject_associations
AS $$
DECLARE
BEGIN
RETURN QUERY
WITH RECURSIVE t
AS
(
SELECT *
FROM subject_associations sa
WHERE sa.id = start_id
UNION ALL
SELECT next.*
FROM t prev
JOIN subject_associations next ON (next.parentid = prev.id)
)
SELECT * FROM t;
END;
$$ LANGUAGE PLPGSQL;

Answer 3

Try this
--- Table
-- DROP SEQUENCE public.data_id_seq;
CREATE SEQUENCE "data_id_seq"
INCREMENT 1
MINVALUE 1
MAXVALUE 9223372036854775807
START 1
CACHE 1;
ALTER TABLE public.data_id_seq
OWNER TO postgres;
CREATE TABLE public.data
(
id integer NOT NULL DEFAULT nextval('data_id_seq'::regclass),
name character varying(50) NOT NULL,
label character varying(50) NOT NULL,
parent_id integer NOT NULL,
CONSTRAINT data_pkey PRIMARY KEY (id),
CONSTRAINT data_name_parent_id_unique UNIQUE (name, parent_id)
)
WITH (
OIDS=FALSE
);
INSERT INTO public.data(id, name, label, parent_id) VALUES (1,'animal','Animal',0);
INSERT INTO public.data(id, name, label, parent_id) VALUES (5,'birds','Birds',1);
INSERT INTO public.data(id, name, label, parent_id) VALUES (6,'fish','Fish',1);
INSERT INTO public.data(id, name, label, parent_id) VALUES (7,'parrot','Parrot',5);
INSERT INTO public.data(id, name, label, parent_id) VALUES (8,'barb','Barb',6);
--- Function
CREATE OR REPLACE FUNCTION public.get_all_children_of_parent(use_parent integer) RETURNS integer[] AS
$BODY$
DECLARE
process_parents INT4[] := ARRAY[ use_parent ];
children INT4[] := '{}';
new_children INT4[];
BEGIN
WHILE ( array_upper( process_parents, 1 ) IS NOT NULL ) LOOP
new_children := ARRAY( SELECT id FROM data WHERE parent_id = ANY( process_parents ) AND id <> ALL( children ) );
children := children || new_children;
process_parents := new_children;
END LOOP;
RETURN children;
END;
$BODY$
LANGUAGE plpgsql VOLATILE COST 100;
ALTER FUNCTION public.get_all_children_of_parent(integer) OWNER TO postgres
--- Test
SELECT * FROM data WHERE id = any(get_all_children_of_parent(1))
SELECT * FROM data WHERE id = any(get_all_children_of_parent(5))
SELECT * FROM data WHERE id = any(get_all_children_of_parent(6))