PostgreSQL recursive parent/child query

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;