querying a hierarchical mysql table in php - php

Question

i have a table with 3 columns id,name and parent_id representing categories. The root categories(categories with no parents) have parent_id 0. All other categories have parent_id as the id of their immediate parent. There is no limit on the depth of categories i mean that a category can be 3,4 or even 10 levels down from a root category. What i need now is a PHP multi-dimensional array that contains all the root categories at the first level and then their immediate sub-categories at the next level,each sub-category under its parent category,and their sub-categories under them 1 level down. So its a tree like structure. there can be many levels in the tree
I dont need the exact code but need an idea. one such idea is get all the root categories with a select query and then fire a select query for each root query to get its subcategoies and so on recursively but that would be too much select queries.
Or if i know that my table is going to contain say 300 rows at the maximum, how can i do something like
$categories=GetResultAsArray(select * from categories);
and now manipulate the $categories array in memory to get the desired tree.

Answer 1

You're right, using the solution with a "parentid" column is simple but it makes you write recursive queries. There are several other designs for storing hierarchical data in a database that let you do queries more efficiently.
See:
What is the most efficient/elegant way to parse a flat table into a tree?
My other answers to SO questions on hierarchical-data
My presentation Models for Hierarchical Data with SQL and PHP
My book SQL Antipatterns Volume 1: Avoiding the Pitfalls of Database Programming
Sql Antipatterns Strike Back

Answer 2

It's possible duplicate (http://stackoverflow.com/questions/8431463/more-efficient-hierarchy-system/8431551) but here is a snippet to query an adjacent tree (parent_id,id,title):
$q = mysql_query("SELECT id, parent_id, name FROM categories");
while ($r = mysql_fetch_row($q)) {
$names[$r[0]] = $r[2];
$children[$r[0]][] = $r[1];
}
function render_select($root=0, $level=-1) {
global $names, $children;
if ($root != 0)
echo '<option>' . strrep(' ', $level) . $names[$root] . '</option>';
foreach ($children[$root] as $child)
render_select($child, $level+1);
}
echo '<select>';
render_select();
echo '</select>';

Answer 3

You can be inspired by this solution that retrieves the breadcrumb trail of a url and its parents (infinite depth of hierarchy levels).
$q = "SELECT T2.*
FROM (
SELECT
#r AS parent_id,
(SELECT #r := `parent`
FROM `route` WHERE id = parent_id)
AS `parent`, #l := #l + 1 AS `depth`
FROM
(SELECT #r := $route_id, #l := 0) vars, `route` T3
WHERE #r <> 0) T1
JOIN `route` T2
ON T1.parent_id = T2.id
ORDER BY T1.`depth` DESC";
if($res = $db->query($q)) {
if($res = $res->fetchAll(PDO::FETCH_ASSOC)) {
if($size = sizeof($res) > 0) {
// push results in breadcrumb items array
$breadcrumb['items'] = $res;
$breadcrumb['levels'] = $size;
// retrieve only titles
$titles = array_column($res, 'title');
// construct html result seperated by '>' or '/'
$breadcrumb['html']['gt'] = implode(' > ', $titles);
$breadcrumb['html']['sl'] = implode(' / ', $titles);
}
}
}
Entire get_breadcrumb() function available here : https://stackoverflow.com/a/63578607/2282880