создать таблицу, она должна выглядеть следующим образом
DROP TABLE IF EXISTS `parent_child`;
CREATE TABLE `parent_child` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`parent_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=latin1;
insert into `parent_child`(`id`,`name`,`parent_id`)
values (1,'cat1',0),(2,'subcat1',1),
(3,'sub-subcat1',2),(4,'sub-subcat2',2),
(5,'cat2',0);
Создать функцию для получения родительского дочернего элемента
DELIMITER $$
USE `yourdatabase`$$
DROP FUNCTION IF EXISTS `GetAllNode1`$$
CREATE DEFINER=`root`@`localhost` FUNCTION `GetAllNode1`(GivenID INT) RETURNS TEXT CHARSET latin1
DETERMINISTIC
BEGIN
DECLARE rv,q,queue,queue_children TEXT;
DECLARE queue_length,front_id,pos INT;
SET rv = '';
SET queue = GivenID;
SET queue_length = 1;
WHILE queue_length > 0 DO
SET front_id = queue;
IF queue_length = 1 THEN
SET queue = '';
ELSE
SET pos = LOCATE(',',queue) + 1;
SET q = SUBSTR(queue,pos);
SET queue = q;
END IF;
SET queue_length = queue_length - 1;
SELECT IFNULL(qc,'') INTO queue_children
FROM (SELECT GROUP_CONCAT(id) AS qc
FROM `parent_child` WHERE `parent_id` = front_id) A ;
IF LENGTH(queue_children) = 0 THEN
IF LENGTH(queue) = 0 THEN
SET queue_length = 0;
END IF;
ELSE
IF LENGTH(rv) = 0 THEN
SET rv = queue_children;
ELSE
SET rv = CONCAT(rv,',',queue_children);
END IF;
IF LENGTH(queue) = 0 THEN
SET queue = queue_children;
ELSE
SET queue = CONCAT(queue,',',queue_children);
END IF;
SET queue_length = LENGTH(queue) - LENGTH(REPLACE(queue,',','')) + 1;
END IF;
END WHILE;
RETURN rv;
END$$
DELIMITER ;
написать запрос для вывода желаемого
SELECT GetAllNode1(id) FROM parent_child
or
SELECT GetAllNode1(id) FROM parent_child where id =1 //for specific parent's child element