PHP/MySQL - building a nav menu hierarchy

https://stackoverflow.com/questions/477793

20-08-2019
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Question

So the final menu will look something like this:

Item B
    Item B-1
        Item B-1-2
        Item B-1-1
Item A
    SubItem A-1
    SubItem A-2
Item C

Based on the following DB records:

id        menu_title          parent_menu_id    menu_level    weight
1         Item A                0                           1                     1
2         Item B                0                           1                     0
3         Item C                0                           1                     2
4         SubItem A-2       1                           2                     1
5         Item B-1             2                           2                     0
6         Item B-1-1          5                           3                     1
7         SubItem A-1       1                           2                     0
8         Item B-1-2          5                           3                     0

How would I go about displaying? My guess is it'll involve storing all the items into a multidimensional array, then looping through it somehow...

Solution

Dealing with the data structure as you have it will often involve recursion or multiple queries to build the tree.

Have you considered other ways of storing a hierarchy? Check out modified pre-order traversal - here's a nice PHP based article about this.

OTHER TIPS

Hierarchical data is somewhat annoying in a relationsal database (excluding Oracle, which has operators in START WITH/CONNECT BY to deal with this). There are basically two models: adjacency list and nested sets.

You've chosen adjacency sets, which is what I typically do too. It's far easier to change than the nested set model, although the nested set model can be retrieved in the correct order in a single query. Adjacency lists can't be. You'll need to build an intermediate data structure (tree) and then convert that into a list.

What I would do (and have done recently in fact) is:

select the entire menu contents in one query ordered by parent ID;
Build a tree of the menu structure using associative arrays or classes/objects;
Walk that tree to create nested unordered lists; and
Use a jQuery plug-in like Superfish to turn that list into a menu.

You build something like this:

$menu = array(
  array(
    'name' => 'Home',
    'url' => '/home',
  ),
  array(
    'name' => 'Account',
    'url' => '/account',
    'children' => array(
      'name' => 'Profile',
      'url' => '/account/profile',
    ),
  ),
  // etc
);

and convert it into this:

<ul class="menu">;
  <li><a href="/">Home</a></li>
  <li><a href="/account">Account Services</a>
    <ul>
      <li><a href="/account/profile">Profile</a></li>
...

The PHP for generating the menu array from is reasonably straightforward but a bit finnicky to solve. You use a recursive tree-walking function that builds the HTML nested list markup but will leave it's implementation as an exercise for the reader. :)

The way your storing hierarchical data isn't as efficient as you might want. I read the article Managing Hierarchical Data in MySQL a few years ago and have since found it as the best solution to managing hierarchy based data in SQL. Next best benefit is that I believe you can grab the entire tree with one query.

I just posted in a similar question my own approach to transform MySQL hierarchical data (adjacency list) into a menu (HTML)

It does not use recursion. And it requires a single query to the database.