-- WHERE `id_group`=7
--
-- |-------------------------| 
-- |        field_value      | 
-- |----|----------|---------| 
-- | id | id_group | id_prev |
-- |----|----------|---------| 
-- |  4 |    7     |   NULL  |
-- |  5 |    7     |    4    |
-- |  6 |    7     |    5    |
-- |  7 |    7     |    6    |
-- |----|----------|---------| 
-- 
-- 4 -> 5 -> 6 -> 7 (Limit 10,20)

DELIMITER //
CREATE PROCEDURE get_max_levels()
BEGIN
SET @lowest_parent := 
 (SELECT MAX(parent) FROM People WHERE parent IS NOT NULL);
SET @levels := 1;
SET @current_parent = @lowest_parent;
WHILE @current_parent IS NOT NULL DO
 SET @current_parent := 
 (SELECT parent FROM People WHERE person_id = @current_parent);
 SET @levels := @levels + 1;
END WHILE;
SELECT @levels;
END //

CREATE TABLE IF NOT EXISTS `test_tb` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `id_group` int(11) NOT NULL,
  `id_prev` int(11) DEFAULT NULL,
  `title` varchar(255) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `id_prev` (`id_prev`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=8 ;

INSERT INTO `test_tb` (`id`, `id_group`, `id_prev`, `title`) VALUES
(1, 1, NULL, 'Первый'),
(2, 2, NULL, '#1'),
(3, 2, 2, '#2'),
(4, 1, 7, 'Пятый'),
(5, 1, 1, 'Второй'),
(6, 1, 5, 'Третий'),
(7, 1, 6, 'Четвертый');

    public function getChain($group){
        $id=$group;
        $index=1;
        while($id){
            // SELECT * FROM `test_tb` WHERE `id_prev`=`id` AND `id_group`=$group
            $id=$this->findByAttributes(array(
                'id_prev'=>$id->id,
                'id_group'=>$group,
            ));
            if($id){
                $result[]=$id;
            }
        }
        return $result;
    }

$Chain=getChain(1);

(1, 1, NULL, 'Первый')
(5, 1, 1, 'Второй')
(6, 1, 5, 'Третий')
(7, 1, 6, 'Четвертый')
(4, 1, 7, 'Пятый')