• adjacency list model
• modified tree preorder traversal (also called nested set model)

In this post I will show you how to use the adjacency list model and how to implement it in PHP. Look at the picture to get a better idea about what the adjacency model looks like:

Adjacency list model

As you can see the hierarchy is created by parent-child relationships. A typical example would be categories in WordPress where every category can have zero or one parent.

How is this achieved in the database? The table just need to have an additional column with the id of the parent. In addition, this column should allow NULL values. For instance:

CREATE TABLE categories (
id INT NOT NULL AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
parent_id INT NULL DEFAULT NULL,
FOREIGN KEY (parent_id) REFERENCES categories(id) ON UPDATE CASCADE,
INDEX (parent_id),
PRIMARY KEY (id)
) ENGINE = INNODB;

Let’s populate the table with some content we can work with:

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);

Adding, editing or removing categories from this table is easy. The query to select the full tree is more complex though:

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;

PHP implementation

Now the intriguing part. You will usually want to display the tree in a nested unordered XHTML list. There are many ways to do it, here is my approach:

Disadvantages

• limited hierarchy levels (not a big problem as you can easily modify the SQL query for any amount of levels)
• complex and long-winded PHP implementation

Advantages

• fast (you need only a single SQL query – much much faster than the usual recursion method)
• very simple hierarchy modifications (you need to update only a single table row each time – not the whole table as in the nested set model)