Multiple relations parent/child with multiple levels - mysql

Question

I have a MySQL table named companies like this:
+---------+-----------+-----------+
| id_comp | comp_name | id_parent |
+---------+-----------+-----------+
| 1 | comp1 | NULL |
| 2 | comp2 | 1 |
| 3 | comp3 | 2 |
| 4 | comp4 | 2 |
| 5 | comp5 | 2 |
| 6 | comp6 | 1 |
| 3 | comp3 | 6 |
| 5 | comp5 | 6 |
| 7 | comp7 | 6 |
| 4 | comp4 | 6 |
| 8 | comp8 | 4 |
+---------+-----------+-----------+
Each company may have multiple parents (ex: comp3, which is child of comp2 and comp6), each parent may have multiple childs and each child can be a parent itself of multiple childs and so on... So, it can have unlimited levels (relations).
I researched several solutions (http://www.codeproject.com/Articles/818694/SQL-queries-to-manage-hierarchical-or-parent-child, http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/), but I don't think it fits my the problem since the same company (based on id_comp column) can have multiple parents.
I have two questions regarding this:
Is this the right approach if I have thousands of relations (scalable) ?
How do I, given a name (which is unique, based on id_comp) query to select its brothers (same parent_id), its direct parent(s), and its direct child(s).

Answer 1

You have a simple "many:many" relationship. However, you have a restriction that is not really relevant (nor checkable) in that there are no loops.
CREATE TABLE Relations (
id_comp ...,
id_parent ...,
PRIMARY KEY(id_comp, id_parent), -- for reaching "up"
INDEX(id_parent, id_comp) -- for reaching "down"
) ENGINE=InnoDB;
This will scale to millions, probably billions, of relations. Since a PRIMARY KEY is, by definition, UNIQUE and an INDEX, it prevents duplicate relations (1 is a parent of 2 only once) and provides an efficient way to traverse one direction.
Use DISTINCT instead of GROUP BY when necessary. Do not use IN ( SELECT ...), it tends to be slow.
My Siblings:
SELECT DISTINCT their_kids.*
FROM Relations AS me
JOIN Relations AS my_parents ON my_parents.id_comp = me.id_parent
JOIN Relations AS their_kids ON their_kids.id_parent = parents.id_comp
WHERE me.id_comp = #me
AND their_kids.id_comp != #me;
My (immediate) Parents:
SELECT my_parents.*
FROM Relations AS me
JOIN Relations AS my_parents ON my_parents.id_comp = me.id_parent
WHERE me.id_comp = #me;
My (immediate) Children:
SELECT my_kids.*
FROM Relations AS me
JOIN Relations AS my_kids ON my_kids.id_parent = me.id_comp
WHERE me.id_comp = #me;
Aunts, uncles, first cousins would be a bit messier. All ancestors or descendants would be much messier, and should be done with a loop in application code or a Stored Procedure.

Answer 2

Mysql isn't the best choice if you need to work with hierarchical data (getting all ancestors/descendants can be tricky). But if all you care about is finding direct parents/children, your table should be fine (although I might break it out into separate Company and CompanyParent tables so that the company name isn't entered multiple times).
This would give you brothers:
select name
from companies
where id_parent in (select id_parent from companies where id_comp = #company_id)
and id_comp <> #company_id
group by name;
This would give you direct parents:
select p.name
from companies p
join companies c on p.id = c.id_parent
where c.id_comp = #company_id
group by c.name;
This would give you direct children:
select c.name
from companies p
join companies c on p.id = c.id_parent
where p.id_comp = #company_id
group by c.name;