Really quick PHP/SQL question

function Vz_subcategories($str = '')
    {
        global $TMPL, $DB;

        $group_id = $TMPL->fetch_param('id');

        $results = $DB->query("SELECT cat_id FROM exp_categories WHERE group_id = '".$group_id."'");

        if ($results->num_rows > 0)
        {
            foreach ($results->result as $row)
            {
                $cats[] = $row['cat_id'];
            }
        }

         $this->return_data = implode('|', $cats);
    }