A more dynamic way of nesting multi-level categories

I am trying to create a way to retrieve and display an endless amount of nested categories.

Now I could select them like this (not tested, not important):

$resulttable1 = mysql_query("SELECT name, id FROM categories WHERE childof=0");
while($rowtable1 = mysql_fetch_array($resulttable1)){ 
$cat1 = $rowtable['name']; 

$resulttable2 = mysql_query("SELECT name, id FROM categories WHERE childof=$rowtable1[id]");
while($rowtable2 = mysql_fetch_array($resulttable2)){ 
$cat2 = $rowtable2['name']; 

$resulttable3 = mysql_query("SELECT name, id FROM categories WHERE childof=$rowtable2[id]");
while($rowtable3 = mysql_fetch_array($resulttable3)){ 
$cat3 = $rowtable3['name'];
}
}
}

But what if a user wants more than 3 "levels" of nesting? How can I make the mysql SELECT in a way that it retrieves an endless amount of nested categories?

UPDATE: Ok, using the link paul provided I got it done like this:

It's not really important how the lft and rgt fields work as they get updated automatically when you insert and remove categories. It's interesting to find out, though. Also, the f开发者_开发百科irst entry is a static value. It's basically just the start of the tree, just leave it in as-is. My script below doesn't echo it based on it's title "products".

<?php
include_once("config.php");

display_tree('products');

function display_tree($root) {  

echo'<table>';

    // retrieve the left and right value of the $root node  
    $result = mysql_query('SELECT lft, rgt FROM tree '.  
                           'WHERE title="'.$root.'";');  
    $row = mysql_fetch_array($result);  

    // start with an empty $right stack  
    $right = array();  

    // now, retrieve all descendants of the $root node  
    $result = mysql_query('SELECT title, lft, rgt FROM tree '.  
                           'WHERE lft BETWEEN '.$row['lft'].' AND '.  
                           $row['rgt'].' ORDER BY lft ASC;');  

    // display each row  
    while ($row = mysql_fetch_array($result)) {
        // only check stack if there is one  
        if (count($right)>0) {  
            // check if we should remove a node from the stack  
            while ($right[count($right)-1]<$row['rgt']) {  
                array_pop($right);  
            }  
        }  
        // display indented node title  
        $repeatamount = (count($right)) - 1;
        if($repeatamount < 0){ $repeatamount = 0; }

        if($row['title'] != 'products'){
        echo'<tr>';
        echo "<td>".str_repeat('-->',$repeatamount ).$row['title']."</td>";  
        echo'</tr>';
        }

        // add this node to the stack  
        $right[] = $row['rgt'];  
    }
echo'</table>';
}
?>

Which will display something like:

And this is an example for inserting a new category: addnew.php?parent=3&title=Shooter which would add the category "Shooter" under "PC" (which is in turn under "Games").

<?php
include_once("config.php");

if(isset($_GET['parent']) && is_numeric($_GET['parent']) && isset($_GET['title']))
{
    $parent = $_GET['parent'];
    $title = mysql_real_escape_string($_GET['title']);
    mysql_query("INSERT INTO tree (parent, title) VALUES ('$parent', '$title')");
    rebuild_tree(0, 0);
}

function rebuild_tree($parent, $left) {     
        // the right value of this node is the left value + 1     
        $right = $left+1;     

        // get all children of this node     
        $result = mysql_query('SELECT id FROM tree '.     
                               'WHERE parent="'.$parent.'";');     
        while ($row = mysql_fetch_array($result)) {     
            // recursive execution of this function for each     
            // child of this node     
            // $right is the current right value, which is     
            // incremented by the rebuild_tree function     
            $right = rebuild_tree($row['id'], $right);     
        }     

        // we've got the left value, and now that we've processed     
        // the children of this node we also know the right value     
        mysql_query('UPDATE tree SET lft='.$left.', rgt='.     
                     $right.' WHERE id="'.$parent.'";');     

        // return the right value of this node + 1     
        return $right+1;     
    }

    ?>  

I hope that helps anyone else looking for the same thing.

If you use childof = 0 to show that the category is a child of no-one you would be able to go backwards until you reached 0. So starting at:

id  name           childof
7   Motor-Racing   6

You would have a while loop that would continue until you got a rowtable['childof'] value of 0.