Codeigniter - selecting children and parents from db - php

Question

I want to pull from my database records corresponding to parent_id, like this:
function getChildren($id, $parent_id) {
$q = $this->db->select('id, name, slug, plat');
$q = $this->db->from('games');
$q = $this->db->where('parent_id',$id);
$q = $this->db->or_where('id',$parent_id);
$q = $this->db->get();
return $q->result_array();
}
It - if it's a children game - get parent_id and search for a game with such id and for other games that has parent_id same as this one. If it's the parent game, it only looks for games with parent_id same as it's id.
The problem is... it's not always working. I have four games in db:
id | parent_id | title
15 | 0 | Abe
19 | 15 | Abe
20 | 0 | RE2
21 | 20 | RE2 DS
First two works, last two - only children (id = 21) shows parent.

Answer 1

You likely could not do that with a relation database. RDBMS are not intended to manage any form of trees.
You can in some simple case, like one-level hierarchy, but as soon as it becomes more complex, it's getting messier and messier.
Keeping your structure, you have to make ONE JOIN per LEVEL, and that means knowing the depth in advance.
A solution to store trees in database is called Nested Tree, it basically stores interval for in each rows, but it is a bit complex to implement by yourself.
Take a look at Wikipedia explanation. There are however library which allows you to programmatically abstract such operations.

Answer 2

Use this query
select
*
from games as g
where parent_id = 0
union all
select
l.*
from games r
left join (select * from games) as l on r.id = t.id
where
r.parent_id != 0

Answer 3

Just after posting my problem, I tried another solution. Funny, I sit with this for more than an hour and after wasting your time, I came with the solution ;)
Anyway, here it is:
function getChildren($id, $parent_id) {
$q = $this->db->select('id, name, slug, plat');
$q = $this->db->from('games');
$q = $this->db->where('parent_id',$id);
$q = $this->db->or_where('id',$parent_id);
$q = $this->db->or_where('parent_id',$parent_id);
$q = $this->db->where('id !=', $id);
$q = $this->db->get();
return $q->result_array();
}