PostgreSQL get parent categories from table - postgresql

Question

I have the table as like below.
CREATE TABLE my.categories (id bigint, parent_id bigint, name varchar(128));
INSERT INTO my.categories (id, parent_id, name) VALUES (1, null, 'LEVEL 1');
INSERT INTO my.categories (id, parent_id, name) VALUES (2, 1, 'LEVEL 2.1');
INSERT INTO my.categories (id, parent_id, name) VALUES (3, 1, 'LEVEL 2.2');
INSERT INTO my.categories (id, parent_id, name) VALUES (4, 2, 'LEVEL 3.1.1');
INSERT INTO my.categories (id, parent_id, name) VALUES (5, 2, 'LEVEL 3.1.2');
INSERT INTO my.categories (id, parent_id, name) VALUES (6, 3, 'LEVEL 3.2.1');
+----+-----------+---------------+
| id | parent_id | name |
+----+-----------+---------------+
| 1 | null | 'LEVEL 1' |
| 2 | 1 | 'LEVEL 2.1' |
| 3 | 1 | 'LEVEL 2.2' |
| 4 | 2 | 'LEVEL 3.1.1' |
| 5 | 2 | 'LEVEL 3.1.2' |
| 6 | 3 | 'LEVEL 3.2.1' |
+----+-----------+---------------+
I need to get all id's for parent categories.
WITH RECURSIVE tree(theId) AS (
SELECT id
FROM my.categories
WHERE id = theId -- wrong here, because its not a procedure
UNION ALL
SELECT table1.id
FROM my.categories AS table1
JOIN tree AS parent ON theId = table1.parent_id
)
SELECT DISTINCT theId FROM tree WHERE theId = 6;
Example result with data but actually I need only id's.
+----+-----------+---------------+
| id | parent_id | name |
+----+-----------+---------------+
| 1 | null | 'LEVEL 1' |
| 3 | 1 | 'LEVEL 2.2' |
| 6 | 3 | 'LEVEL 3.2.1' |
+----+-----------+---------------+
Or like this:
+----+-----------+---------------+
| id | parent_id | name |
+----+-----------+---------------+
| 3 | 1 | 'LEVEL 2.2' |
| 6 | 3 | 'LEVEL 3.2.1' |
+----+-----------+---------------+
The trouble is I'm not allowed to use procedures. This query should be used as sub-query for many other queries. And please dont look at name column it is irrelevant.

Answer 1

If I get you, this is what you need.
First, with the folowing query you can get all the parent ids:
WITH RECURSIVE t(id, parentlist) AS (
SELECT id, ARRAY[]::bigint[] FROM my.categories WHERE parent_id IS NULL
UNION
SELECT my.categories.id, my.categories.parent_id || t.parentlist
FROM my.categories
JOIN t ON categories.parent_id = t.id
) SELECT * FROM t
-- outputs:
-- id | parentlist
-- ----+------------
-- 1 | {}
-- 2 | {1}
-- 3 | {1}
-- 4 | {2,1}
-- 5 | {2,1}
-- 6 | {3,1}
If you want to get a record of the parents of one id you just need to change the query like:
WITH RECURSIVE t(id, parentlist) AS (
SELECT id, ARRAY[]::bigint[] FROM my.categories WHERE parent_id IS NULL
UNION
SELECT my.categories.id, my.categories.parent_id || t.parentlist
FROM my.categories
JOIN t ON categories.parent_id = t.id
) SELECT unnest(parentlist) as parents_ids FROM t WHERE id=6;
-- outputs:
-- parents_ids
-- -----------
-- 3
-- 1
Note that the last query does not output the "current" id (6).