Multidimensionale matrix om categorieën en subcategorieën te groeperen

U moet eerst alle subcategorieën en categorieën op één zoekopdracht krijgen:

SQL:

SELECT     sub_category_id, 
           category_id,
           sub_category_name,
           category_name           
FROM       sub_categories a
INNER JOIN categories b
    ON     a.category_id=b.category_id

PHP:

$categories = array();
while ($row = mysql_fetch_assoc($result)) {
    $cat_name = $row['category_name'];
    $sub_cat_id = $row['sub_category_id'];
    $sub_cat_name = $row['sub_category_name'];
    $categories[$cat_name][$sub_cat_id] = $sub_cat_name;
}

var_dump($categories);