Create the N-level Categories tree with single SQL query using CI

To create the N-level Categories tree with single SQL query using CI

class Category_model extends Model {

 private $table = 'categories';
 private $selectTreeOptions_data = null;
 private $selectTreeOptions_index = null;
 
 public function get_categories() {
  $this->db->select('c.id, c.parent_id, c.name');
  $this->db->from($this->table .' AS c');
  $this->db->where('c.status', '1');
  
  $query = $this->db->get();
  //$str = $this->db->last_query();
  
  return $query->result_array();
 }
 
 /*
  * Recursive top-down tree traversal example:
  * Indent and print child nodes
  */
 function getSelectTreeOptions($parent_id, $level, $selected_id='')
 {
  $html = '';
     $data = $this->selectTreeOptions_data;
     $index = $this->selectTreeOptions_index;
     $parent_id = $parent_id === NULL ? 0 : $parent_id;
    
     if (isset($index[$parent_id])) {
         foreach ($index[$parent_id] as $id) {
          $selected = isset($selected_id) && ($selected_id==$data[$id]["id"]) ? 'selected="selected"' : '';
             $html .= ' '.str_repeat("-", $level) .' '. $data[$id]["name"] . " (".$data[$id]["id"].")\n";
             $html .= $this->getSelectTreeOptions($id, $level + 1, $selected_id);
         }
     }
          
     return $html;
 }

 // Get the category tree with select options.
 function get_categories_tree_options($selected_id='')
 {
   $categories = $this->get_categories();
  $this->data = '';
  $this->index = '';
  
  foreach ($categories as $row) :
      $id = $row["id"];
      $parent_id = $row["parent_id"] === NULL ? "NULL" : $row["parent_id"];
      $this->selectTreeOptions_data[$id] = $row;
      $this->selectTreeOptions_index[$parent_id][] = $id;
  endforeach;
  
  $items = $this->getSelectTreeOptions(NULL, 0, $selected_id);
  
  return $items;  
 }
 
}

Category_model::get_categories_tree_options($selected_id='');