find out list of inverted hierarchy in mysql - mysql

Question

Imagine I have a table (Mysql myISAM) with a child->parent relationship (categories and multiple levels of subcategories)
+--------+---------+
| id |parent_id|
+--------+---------+
| 1 | null |
| 2 | 1 |
| 3 | 2 |
| 4 | 7 |
| 5 | 1 |
| 6 | 5 |
+--------+---------+
How would you find all children of some ID, like querying for id 1 would output :
2,5,3,6 ? (order has no importance)
So in other words, how to do a reverted children lookup on this parent_link ?
At the moment, I cycle in php and query for the parent_id, then again and concatenate all the results in a string while there are results, but this is so slow...

Answer 1

Ok, so thanks to Deepak code, I managed to write this, a bit shorter readable, it accepts a table as parameter and returns also the depth of the element.
DELIMITER $$
CREATE PROCEDURE get_children(IN V_KEY INT,IN SOURCETABLE VARCHAR(255))
proc:
BEGIN
DECLARE vid text;
DECLARE count int;
DROP TABLE IF EXISTS `temp_child_nodes`;
CREATE TEMPORARY TABLE temp_child_nodes(id int, depth int);
SET vid = V_KEY;
SET #val = '';
SET count = 0;
WHILE (vid is NOT NULL) DO
SET #sql = CONCAT("INSERT INTO temp_child_nodes(id,depth) SELECT id,'",count,"' from ",SOURCETABLE," where parent_id IN (",vid,")");
PREPARE stmt1 FROM #sql;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
SET #tsql = CONCAT("SELECT GROUP_CONCAT(id) INTO #val from ",SOURCETABLE," where parent_id IN (", vid, ")");
PREPARE stmt2 FROM #tsql;
EXECUTE stmt2;
DEALLOCATE PREPARE stmt2;
SET vid = #val;
SET count = count + 1;
END WHILE;
#output data
SELECT * from temp_child_nodes;
END
$$
DELIMITER ;

Answer 2

create table my_table(
id int,
parent_id int
);
insert into my_table values
(1,null),
(2,1),
(3,2),
(4,7),
(5,1),
(6,5);
This stored procedure will get you all the children of any given id
DELIMITER $$
DROP PROCEDURE IF EXISTS get_children$$
CREATE PROCEDURE get_children(IN V_KEY INT)
proc:
BEGIN
DECLARE vid text;
declare oid text;
DECLARE count int;
CREATE TEMPORARY TABLE temp_child_nodes(
id int
);
SET vid = V_KEY;
INSERT INTO temp_child_nodes(id) SELECT id from my_table where parent_id = vid;
SELECT GROUP_CONCAT(concat("'",id,"'")) INTO oid from my_table where parent_id = vid;
SET vid = oid;
SET count = 0;
SET #val = '';
WHILE (vid is NOT NULL) DO
SET #sql = CONCAT("INSERT INTO temp_child_nodes(id) SELECT id from my_table where parent_id IN (",vid,")");
PREPARE stmt1 FROM #sql;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
SET #tsql = CONCAT("SELECT GROUP_CONCAT(id) INTO #val from my_table where parent_id IN (", vid, ")");
PREPARE stmt2 FROM #tsql;
EXECUTE stmt2;
DEALLOCATE PREPARE stmt2;
SET vid = #val;
SET count = count + 1;
END WHILE;
#SELECT count;
SELECT * from temp_child_nodes;
#SELECT oid;
END
$$
DELIMITER ;
CALL get_children(1);
mysql> CALL get_children(1);
+------+
| id |
+------+
| 2 |
| 5 |
| 3 |
| 6 |
+------+
4 rows in set (0.22 sec)
Query OK, 0 rows affected (0.22 sec)

Answer 3

Here is the sqlfiddle demo for your query
http://sqlfiddle.com/#!2/ca90e/6
if there can be 'n' number of child then you need to use a stored procedure