But when I wrote this I was working on a simple CMS application where the client would hardly ever create more than 1 level of hierarchy (so 4 levels were more than enough). And many applications are like that – with just one or two levels of hierarchy.

If you need unlimited levels then, of course, either use the nested set model or the adjacency list model the way you are suggesting.

Thanks for comments

For what it’s worth, I think @ficuscr was after a PHP function that uses recursion to return a nested structure from a SELECT * scenario like I described in my previous comment. Basically a findChildren( $cat ) method that calls itself to ouput something like:

$cats = array (
‘id’ => 1 ,
‘name’ => ‘Top 1′,
‘children’ => array (
‘id’ => 2,
‘name’ => ‘Sub 1′,
‘children’ => array(
‘id’ => 3
‘name’ => ‘Third 1′,
‘children => array()
)
)
)

If you have that, then you can use recursive function to output a nested ul li as well. The benefit to using recursive functions is that they’d work to any arbitrary depth.

What I’m looking for is SQL that renders something like this:

1 | Top 1 | Null | Null
2 | Top 1 | Sub 1 | Null
3 | Top 1 | Sub 1 | Third 1
4 | Top 1 | Sub 2 | Null
5 | Top 2 | Null | Null
6 | Top 2 | Sub 3 | Null

… just like you would logically think of how a menu would be constructed.

Great blog, by the way! Lots of really useful stuff here. Keep going.

CREATE TABLE categories (
id INT NOT NULL AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
parent_id INT NULL DEFAULT NULL,
INDEX (parent_id),
PRIMARY KEY (id)
) ENGINE = MYISAM;

Then:

INSERT INTO categories (id, name, parent_id) VALUES
(NULL, ‘Relational databases’, NULL),
(NULL, ‘SQLite’, 1),
(NULL, ‘MySQL’, 1),
(NULL, ‘PostgreSQL’, 1),
(NULL, ‘PDO’, 3),
(NULL, ‘Transactions’, 3);

Then:

SELECT t0.id AS lvl0_id, t0.name AS lvl0_name,
t1.id AS lvl1_id, t1.name as lvl1_name,
t2.id AS lvl2_id, t2.name as lvl2_name,
t3.id AS lvl3_id, t3.name as lvl3_name,
t4.id AS lvl4_id, t4.name as lvl4_name
FROM categories AS t0
LEFT JOIN categories AS t1 ON t1.parent_id = t0.id
LEFT JOIN categories AS t2 ON t2.parent_id = t1.id
LEFT JOIN categories AS t3 ON t3.parent_id = t2.id
LEFT JOIN categories AS t4 ON t4.parent_id = t3.id
ORDER BY t0.id, t1.id, t2.id, t3.id, t4.id;

And it works as expected.

Yes, the point of this approach is that it has limited number of levels, so it can only go as deep as the number of levels defined in the SELECT query (I defined 4 levels in this example but you can define 10 or more if you need). But it doesn’t use a recursion which means it is much faster and overall more elegant solution.

If you have a hierarchy with 10+ levels I would advise against using the adjacency list model. In that case use the nested set model.

CREATE TABLE category (
id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
category VARCHAR(100) NOT NULL,
parent_id INT UNSIGNED NOT NULL ,
INDEX( parent_id )
)

but InnoDB & Foreign Key constraints are not required for adjacency SQL?
Don’t you only get rows that have all 4 levels deep?

Here is some alternate SQL that I’ve used in the past but is nowhere near as elegant as the solution I really want to create. It uses MySQL’s WHERE … IN ( ) and sub-select/subquery syntax to find categories in the top level, then level 1, … one SQL query per level is required. Each deeper level requires an additional layer in the sub-select, but it’s pretty easy to follow, and each subquery is based on the previous level.

# LEVEL 0 #################################

SELECT
t0.id as id0,
t0.name as name0,
FROM category t0
WHERE
t0.parent_id IS NULL
ORDER BY
t0.ord

# LEVEL 1 #################################

SELECT
t1.id as id1,
t1.parent_id as parent1,
t1.name as name1,
FROM caegory t1
WHERE
t1.parent_id IN
(
SELECT
t0.id as id0
FROM
category t0
WHERE
t0.parent_pid IS NULL
)
ORDER BY
t1.ord

# LEVEL 2 #################################

SELECT
t2.id as id2 ,
t2.parent_id as parent2 ,
t2.name as name2
FROM
category AS t2
WHERE
t2.parent_id
IN (
SELECT t1.id as id1
FROM category t1
WHERE t1.parent_id IN
(
SELECT t0.id as id0
FROM category t0
WHERE t0.parent_id IS NULL
)
)