MySql: ORDER BY parent and child - mysql

Question

I have a table like:
+------+---------+-
| id | parent |
+------+---------+
| 2043 | NULL |
| 2044 | 2043 |
| 2045 | 2043 |
| 2049 | 2043 |
| 2047 | NULL |
| 2048 | 2047 |
| 2049 | 2047 |
+------+---------+
which shows a simple, 2-level "parent-child"-corelation. How can I ORDER BY an SELECT-statement to get the order like in the list above, which means: 1st parent, childs of 1st parent, 2nd parent, childs of 2nd parent and so on (if I have that, I can add the ORDER BYs for the children... I hope). Is it possible withoug adding a sort-field?

Answer 1

Including sorting children by id:
ORDER BY COALESCE(parent, id), parent IS NOT NULL, id
SQL Fiddle example
Explanation:
COALESCE(parent, id): First sort by (effectively grouping together) the parent's id.
parent IS NOT NULL: Put the parent row on top of the group
id: Finally sort all the children (same parent, and parent is not null)

Answer 2

If your table uses 0 instead of null to indicate an entry with no parent:
id | parent
-------------
1233 | 0
1234 | 1233
1235 | 0
1236 | 1233
1237 | 1235
Use greatest instead of coalesce and check the value does not equal 0:
ORDER BY GREATEST(parent, id), parent != 0, id

Answer 3

The solution above didn't work for me, my table used 0 instead of NULL.
I found this other solution: you create a column with the concatened parent id and child id in your query and you can sort the result by it .
SELECT CONCAT(IF(parent = 0,'',CONCAT('/',parent)),'/',id) AS gen_order
FROM table
ORDER BY gen_order

Answer 4

This question still shows as one of the first search results. So I would like to share a my solution and hope it will help more people out. This will also work when you have a table with many levels of parent and child relations. Although it is quite a slow solution. The top level has NULL as parent.
+---------+---------+
| id | parent |
+---------+---------+
| 1 | NULL |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
+---------+---------+
In my approach I will use a procedure that will recursively call itself and keep prepending the path with the parent of the requested id until it reaches the NULL parent.
DELIMITER $$
CREATE DEFINER=`root`#`localhost` PROCEDURE `PATH`(IN `input` INT, OUT `output` VARCHAR(128))
BEGIN
DECLARE _id INT;
DECLARE _parent INT;
DECLARE _path VARCHAR(128);
SET `max_sp_recursion_depth` = 50;
SELECT `id`, `parent`
INTO _id, _parent
FROM `database`.`table`
WHERE `table`.`id` = `input`;
IF _parent IS NULL THEN
SET _path = _id;
ELSE
CALL `PATH`(_parent, _path);
SELECT CONCAT(_path, '-', _id) INTO _path;
END IF;
SELECT _path INTO `output`;
END $$
DELIMITER ;
To use the results in an ORDER BY clause you will need a FUNCTION too that wraps the results of the PROCEDURE.
DELIMITER $$
CREATE DEFINER=`root`#`localhost` FUNCTION `GETPATH`(`input` INT) RETURNS VARCHAR(128)
BEGIN
CALL `PATH`(`input`, #path);
RETURN #path;
END $$
DELIMITER ;
Now we can use the recursive path to sort the order of the table. On a table with 10000 rows it takes just over a second on my workstation.
SELECT `id`, `parent`, GETPATH(`id`) `path` FROM `database`.`table` ORDER BY `GETPATH`(`id`);
Example output:
+---------+---------+---------------+
| id | parent | path |
+---------+---------+---------------+
| 1 | NULL | 1 |
| 10 | 1 | 1-10 |
| 300 | 10 | 1-10-300 |
| 301 | 300 | 1-10-300-301 |
| 302 | 300 | 1-10-300-302 |
+---------+---------+---------------+
5 rows in set (1,39 sec)