How to build a tree view with PHP / SQL?

What's the best way to:

1. Get the data from the db using a single query
2. Loop through the results building e.g. a nested unordered list

My table has id, name and parent_id columns.

From stackoverflow

• MySQL have created a good article on this subject: Managing Hierarchical Data in MySQL

• Here's an update to my last answer, with a counter that gives each ul a nesting 'level' class, and some comments.

  Could anyone suggest how to adapt this to use table rows, without nesting, but with some kind of class numbering hierarchy for css/js hooks?

  <?
  
  //
  // Get the data
  //
  include_once("inc/config.php");
  
  $query = "SELECT c.* 
            FROM categories AS c
            ORDER BY c.id
            LIMIT 1000";          
  
  $result = pg_query($db, $query);
  
  //
  // Load all the results into the row array
  //
  while ($row = pg_fetch_array($result, NULL, PGSQL_ASSOC))
  {
    //
    // Wrap the row array in a parent array, using the id as they key
    // Load the row values into the new parent array
    //
    $categories[$row['id']] = array(
      'id' => $row['id'], 
      'description' => $row['description'], 
      'parent_id' => $row['parent_id']
    );
  }
  
  
  // print '<pre>';
  // print_r($category_array);
  
  // ----------------------------------------------------------------
  
  //
  // Create a function to generate a nested view of an array (looping through each array item)
  //
  function generate_tree_list($array, $parent = 0, $level = 0)
  {
  
    //
    // Reset the flag each time the function is called
    //
    $has_children = false;
  
    //
    // Loop through each item of the list array
    //
    foreach($array as $key => $value)
    {
      //
      // For the first run, get the first item with a parent_id of 0 (= root category)
      // (or whatever id is passed to the function)
      //
      // For every subsequent run, look for items with a parent_id matching the current item's key (id)
      // (eg. get all items with a parent_id of 2)
      //
      // This will return false (stop) when it find no more matching items/children
      //
      // If this array item's parent_id value is the same as that passed to the function
      // eg. [parent_id] => 0   == $parent = 0 (true)
      // eg. [parent_id] => 20  == $parent = 0 (false)
      //
      if ($value['parent_id'] == $parent) 
      {                   
  
        //
        // Only print the wrapper ('<ul>') if this is the first child (otherwise just print the item)      
        // Will be false each time the function is called again
        //
        if ($has_children === false)
        {
          //
          // Switch the flag, start the list wrapper, increase the level count
          //
          $has_children = true;  
  
          echo '<ul class="level-' . $level . '">';
  
          $level++;
        }
  
        //
        // Print the list item
        //
        echo '<li><a href="?id=' . $value['id'] . '">' . $value['description'] . '</a>';
  
        //
        // Repeat function, using the current item's key (id) as the parent_id argument
        // Gives us a nested list of subcategories
        //
        generate_tree_list($array, $key, $level); 
  
        //
        // Close the item
        //
        echo '</li>';
  
  
      }
  
    }
  
    //
    // If we opened the wrapper above, close it.
    //
    if ($has_children === true) echo '</ul>';
  
  
  }
  
  // ----------------------------------------------------------------
  
  //
  // generate list
  //
  generate_tree_list($categories);
  
  
  ?>
  

  meleyal : Eran Galperin's solution is probably much cleaner, but I just needed something simple I could understand in < 10 mins :)