I have a category system in a MySQL database with parents and childs. The database only stores the id of it''s immediate parent (or 0 if on root). Since the system allows multiple subcategories there are cases of multiple childs.

For example

[98] Storage
    [1] External
        [3] Pendrives
        [4] Portable hhdds
    [2] Internal
        [5] Sata hhdd
        [6] IDE hhdd
[...]
[99] Clothing

The database would be

id    parent_id    name
1     98           External
2     98           Internal
3     1            Pendrives
4     1            Portable
5     2            Sata
6     2            IDE
98    0            Storage
99    0            Clothing

I also have a products table with a category id and I need to get a list of all the products in the first level of categories.

For example:

Product   Category
 A          3
 B          4
 C          5
 D          6
 E          74

Should return 98: A, B, C, D 99: X, Y, Z...

I'm stuck and I can't think of the logic to retrieve it in that way.

I started by getting the IDs of all the categories that aren't in the first level by:

while ($row = mysql_fetch_assoc($result)) {
    if ($row['parent_id'] != 0) {
        $level1[$i]['name'] = utf8_encode($row['categories_name']);
        $level1[$i]['id'] = $row['categories_id'];
    }

    $i++;
}

but I'm having a burnout and can't think of a way that would nest them. I thought some kind of while but it's infinite :P

Any ideas please?