How to select subcategories from selected category using a nested function in PHP?

Question

First of all, I posted another question about this, but didn\'t get answer so I needed to post a new one.

I have a function which is listing categories hierarchicall

Answer 1

There are a couple of solutions. First of all, I'll use the following data (categories table) as an example.

+----+--------------------------+-----------+
| id | name                     | parent_id |
+----+--------------------------+-----------+
|  1 | Electronics              |      NULL |
|  2 | Apparel & Clothing       |      NULL |
|  3 | Phones & Accessories     |         1 |
|  4 | Computer & Office        |         1 |
|  5 | Men's Clothing           |         2 |
|  6 | Women's Clothing         |         2 |
|  7 | Cell Phones              |         3 |
|  8 | Cell Phone Accessories   |         3 |
|  9 | Phone Parts              |         3 |
| 10 | Computers & Accessories  |         4 |
| 11 | Tablets & Accessories    |         4 |
| 12 | Computer Peripherals     |         4 |
| 13 | Computer Components      |         4 |
| 14 | Office Electronics       |         4 |
+----+--------------------------+-----------+

Solution 1 (Adjacency List):

You can easily fetch either all categories or subcategories of a category in a single query using WITH (Common Table Expressions) clause (requires MySQL 8.0):

// Database connection

$options = [
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::ATTR_EMULATE_PREPARES => false,
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
];

$pdo = new PDO('mysql:host=localhost;dbname=<DATABASE_NAME>', '<USERNAME>', '<PASSWORD>', $options);

function getCategories(PDO $db, $parentId = null)
{
    $sql = $parentId ? 'WITH RECURSIVE cte (id, name, parent_id) AS (SELECT id, name, parent_id FROM categories WHERE parent_id = ? UNION ALL SELECT c.id, c.name, c.parent_id FROM categories c INNER JOIN cte ON c.parent_id = cte.id) SELECT * FROM cte' : 'SELECT * FROM categories';
    $stmt = $db->prepare($sql);
    $stmt->execute($parentId ? [$parentId] : null);
    return $stmt->fetchAll();
}

If you're using MySQL 5.7, change that function like so:

function getCategories(PDO $db, $parentId = null)
{
    $sql = $parentId ? 'SELECT id, name, parent_id FROM (SELECT * FROM categories ORDER BY parent_id, id) c, (select @pv := ?) initialisation WHERE find_in_set(parent_id, @pv) AND LENGTH(@pv := concat(@pv, ",", id))' : 'SELECT * FROM categories';
    $stmt = $db->prepare($sql);
    $stmt->execute($parentId ? [$parentId] : null);
    return $stmt->fetchAll();
}

To get all categories in your database:

$allCategories = getCategories($pdo);

Output:

+----+--------------------------+-----------+
| id | name                     | parent_id |
+----+--------------------------+-----------+
|  1 | Electronics              |      NULL |
|  2 | Apparel & Clothing       |      NULL |
|  3 | Phones & Accessories     |         1 |
|  4 | Computer & Office        |         1 |
|  5 | Men's Clothing           |         2 |
|  6 | Women's Clothing         |         2 |
|  7 | Cell Phones              |         3 |
|  8 | Cell Phone Accessories   |         3 |
|  9 | Phone Parts              |         3 |
| 10 | Computers & Accessories  |         4 |
| 11 | Tablets & Accessories    |         4 |
| 12 | Computer Peripherals     |         4 |
| 13 | Computer Components      |         4 |
| 14 | Office Electronics       |         4 |
+----+--------------------------+-----------+

To get subcategories of a category:

$subCategories = getCategories($pdo, 1); // 1 is parent_id

Output:

+----+--------------------------+-----------+
| id | name                     | parent_id |
+----+--------------------------+-----------+
|  3 | Phones & Accessories     |         1 |
|  4 | Computer & Office        |         1 |
|  7 | Cell Phones              |         3 |
|  8 | Cell Phone Accessories   |         3 |
|  9 | Phone Parts              |         3 |
| 10 | Computers & Accessories  |         4 |
| 11 | Tablets & Accessories    |         4 |
| 12 | Computer Peripherals     |         4 |
| 13 | Computer Components      |         4 |
| 14 | Office Electronics       |         4 |
+----+--------------------------+-----------+

If you want an HTML output, you can loop through $allCategories / $subCategories (based on your example):

function prepareCategories(array $categories)
{
    $result = [
        'all_categories' => [],
        'parent_categories' => []
    ];
    foreach ($categories as $category) {
        $result['all_categories'][$category['id']] = $category;
        $result['parent_categories'][$category['parent_id']][] = $category['id'];
    }
    return $result;
}

function buildCategories($categories, $parentId = null)
{
    if (!isset($categories['parent_categories'][$parentId])) {
        return '';
    }

    $html = '<ul>';
    foreach ($categories['parent_categories'][$parentId] as $cat_id) {
        if (isset($categories['parent_categories'][$cat_id])) {
            $html .= "<li><a href='#'>{$categories['all_categories'][$cat_id]['name']}</a>";
            $html .= buildCategories($categories, $cat_id);
            $html .= '</li>';
        } else {
            $html .= "<li><a href='#'>{$categories['all_categories'][$cat_id]['name']}</a></li>";
        }
    }
    $html .= '</ul>';

    return $html;
}

echo buildCategories(prepareCategories($allCategories));

Output:

echo buildCategories(prepareCategories($subCategories), 1);

Output:

Solution 2 (Nested Sets):

We'll add additional columns left and right to our table and put numbers in it which will identify the groups belonging to the parent. (Note that we won't be using parent_id column.)

+----+--------------------------+--------------------------+
| id | name                     | parent_id | left | right |
+----+--------------------------+--------------------------+
|  1 | Electronics              |      NULL |    1 |    22 |
|  2 | Apparel & Clothing       |      NULL |   23 |    28 |
|  3 | Phones & Accessories     |         1 |    2 |     9 |
|  4 | Computer & Office        |         1 |   10 |    21 |
|  5 | Men's Clothing           |         2 |   24 |    25 |
|  6 | Women's Clothing         |         2 |   26 |    27 |
|  7 | Cell Phones              |         3 |    3 |     4 |
|  8 | Cell Phone Accessories   |         3 |    5 |     6 |
|  9 | Phone Parts              |         3 |    7 |     8 |
| 10 | Computers & Accessories  |         4 |   11 |    12 |
| 11 | Tablets & Accessories    |         4 |   13 |    14 |
| 12 | Computer Peripherals     |         4 |   15 |    16 |
| 13 | Computer Components      |         4 |   17 |    18 |
| 14 | Office Electronics       |         4 |   19 |    20 |
+----+--------------------------+--------------------------+

Now, we need to change our function:

function getCategories(PDO $db, $parentId = null)
{
    $sql = $parentId ? 'SELECT children.* FROM categories parent INNER JOIN categories children ON parent.left < children.left AND parent.right > children.left WHERE parent.id = ?' : 'SELECT * FROM categories';
    $stmt = $db->prepare($sql);
    $stmt->execute($parentId ? [$parentId] : null);
    return $stmt->fetchAll();
}

To get all categories in your database:

$allCategories = getCategories($pdo);

Output:

+----+--------------------------+--------------------------+
| id | name                     | parent_id | left | right |
+----+--------------------------+--------------------------+
|  1 | Electronics              |      NULL |    1 |    22 |
|  2 | Apparel & Clothing       |      NULL |   23 |    28 |
|  3 | Phones & Accessories     |         1 |    2 |     9 |
|  4 | Computer & Office        |         1 |   10 |    21 |
|  5 | Men's Clothing           |         2 |   24 |    25 |
|  6 | Women's Clothing         |         2 |   26 |    27 |
|  7 | Cell Phones              |         3 |    3 |     4 |
|  8 | Cell Phone Accessories   |         3 |    5 |     6 |
|  9 | Phone Parts              |         3 |    7 |     8 |
| 10 | Computers & Accessories  |         4 |   11 |    12 |
| 11 | Tablets & Accessories    |         4 |   13 |    14 |
| 12 | Computer Peripherals     |         4 |   15 |    16 |
| 13 | Computer Components      |         4 |   17 |    18 |
| 14 | Office Electronics       |         4 |   19 |    20 |
+----+--------------------------+--------------------------+

To get subcategories of a category:

$subCategories = getCategories($pdo, 1); // 1 is parent_id

Output:

+----+--------------------------+--------------------------+
| id | name                     | parent_id | left | right |
+----+--------------------------+--------------------------+
|  3 | Phones & Accessories     |         1 |    2 |     9 |
|  4 | Computer & Office        |         1 |   10 |    21 |
|  7 | Cell Phones              |         3 |    3 |     4 |
|  8 | Cell Phone Accessories   |         3 |    5 |     6 |
|  9 | Phone Parts              |         3 |    7 |     8 |
| 10 | Computers & Accessories  |         4 |   11 |    12 |
| 11 | Tablets & Accessories    |         4 |   13 |    14 |
| 12 | Computer Peripherals     |         4 |   15 |    16 |
| 13 | Computer Components      |         4 |   17 |    18 |
| 14 | Office Electronics       |         4 |   19 |    20 |
+----+--------------------------+--------------------------+

You can render HTML as shown in Solution 1. Read more about updating and inserting new data in nested set model.

---

Sources & Reads:

• Managing hierarchical data in MySQL - nested set
• Managing Hierarchical Data in MySQL
• Models for hierarchical data
• Managing Hierarchical Data in MySQL Using the Adjacency List Model
• Adjacency list vs. nested sets: MySQL
• One more Nested Intervals vs. Adjacency List comparison