How to find all IDs of children recursively?

Question

I would like to get all IDs from children in a tree with MySQL only.

I have a table like this:

ID parent_id name
1  0         cat1
2  1         subca         


        

Answer 1

create table it should be look like below

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);

Create function for getting parent child element

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 ;

write query for desire output

SELECT GetAllNode1(id) FROM parent_child 
or 
SELECT GetAllNode1(id) FROM parent_child  where id =1 //for specific parent's child element