Postgresql hierarchical (tree) query - sql

Question

I found few topics about it but none fits my expected results.
I have levels of categories stored in the table, just want to display it as tree structure.
All answers are kind of following query:
DB FIDDLE
WITH RECURSIVE cte AS (
SELECT category_id, category_name, parent_category, 1 AS level
FROM category
WHERE level = 1
UNION ALL
SELECT c.category_id, c.category_name, c.parent_category, ct.level + 1
FROM cte ct
JOIN category c ON c.parent_category = ct.category_id
)
SELECT *
FROM cte;
But the results are like
level
1
1
2
2
2
3
3
3
3
3
What I want to achieve is
level
1
2
3
3
2
3
3
1
2
3
3
2
3
3

Answer 1

You would typically keek track of the path to each node and use that for ordering. In Postgres, arrays come handy for this:
with recursive cte as (
select category_id, category_name, parent_category, 1 as level, array[category_id] path
from category
where parent_category is null
union all
select c.category_id, c.category_name, c.parent_category, ct.level + 1, ct.path || c.category_id
from cte ct
join category c on c.parent_category = ct.category_id
)
select *
from cte
order by path
Note that there is no need to store the level in the table; you can compute the information on the fly as you iterate. To identify the root nodes, you can filter on rows whose parent is null.
In your db fiddle, the query returns:
category_id | category_name | parent_category | level | path
----------: | :------------ | --------------: | ----: | :-------
1 | cat1 | null | 1 | {1}
3 | cat3 | 1 | 2 | {1,3}
8 | cat8 | 3 | 3 | {1,3,8}
9 | cat9 | 3 | 3 | {1,3,9}
4 | cat4 | 1 | 2 | {1,4}
6 | cat6 | 4 | 3 | {1,4,6}
7 | cat7 | 4 | 3 | {1,4,7}
5 | cat5 | 1 | 2 | {1,5}
10 | cat10 | 5 | 3 | {1,5,10}
11 | cat11 | 5 | 3 | {1,5,11}
2 | cat2 | null | 1 | {2}

Answer 2

You can keep track of the hierarchy as an array and use that for ordering:
WITH RECURSIVE cte AS (
SELECT category_id, category_name, parent_category, 1 AS level, array[category_id] as categories
FROM category
WHERE level = 1
UNION ALL
SELECT c.category_id, c.category_name, c.parent_category, ct.level + 1, ct.categories || c.category_id
FROM cte ct JOIN
category c
ON c.parent_category = ct.category_id
)
SELECT *
FROM cte
ORDER BY categories;
Here is a db<>fiddle.