Finding root node for every element recursively in SQL - postgresql

Question

Consider the following sample from the table category:
category_id | category_name | super_category
-------------+----------------------------------------------------------------+----------------
1 | Features +|
| |
2 | Alle SACDs +| 1
| |
3 | Formate +|
| |
4 | Box-Sets +| 3
| |
5 | Action, Thriller & Horror +| 4
| |
6 | Alternative +| 4
| |
7 | Blues +| 4
| |
8 | Country +| 4
| |
9 | Alternative Country & Americana& Country | 8
10 | Bestseller& Country | 8
11 | Bluegrass& Country | 8
...
The column super_category lists the immediate parent category for any given category. When super_category is NULL, that category is a main category.
Table with all of the main categories:
SELECT * FROM category WHERE super_category IS NULL;
category_id | category_name | super_category
-------------+----------------+----------------
1 | Features +|
| |
3 | Formate +|
| |
497 | Formats +|
| |
544 | Genres +|
| |
923 | Interpreten +|
| |
941 | Kategorien +|
| |
19208 | Schauspieler +|
| |
19211 | Shop-überblick+|
| |
19502 | Shops +|
| |
21350 | Subjects +|
| |
21513 | Unter 10 EUR +|
| |
21520 | Unter 15 EUR +|
I need to write a recursive query that outputs a table that lists all the categories and which main categories the belong to.
So far I only have the following:
WITH RECURSIVE query AS (
SELECT cat.category_id, cat.super_category
FROM category cat
WHERE cat.super_category IS NULL
UNION ALL
SELECT query.category_id, cat.super_category
FROM query JOIN category cat on cat.super_category = query.category_id
)
SELECT * FROM query;
The logic is as follows:
We define the base case where the category is a main category (super_category IS NULL)
Then we define the recursive case, but I am not sure how I should define it.
Any suggestions?

Answer 1

You are in the correct path with the use of a recursive CTE. You can do:
with recursive
n as (
select category_id, category_name, super_category, 0 as level
from category
union all
select n.category_id, n.category_name, c.super_category, n.level + 1
from n
join category c on c.category_id = n.super_category
)
select category_id, category_name, super_category
from n
where (category_id, level) in (
select distinct on (category_id) category_id, level
from n
order by category_id, level desc
)