handle parent/child categories in mysql/php - php

Question

I have this mysql table "categories":
id category parent
1 category1 NULL
2 subcategory1 1
3 category2 NULL
4 subcategory2 2
and I want to get this result:
category1
subcategory1
subcategory2
category2
to get this result I use this code, but is very slow:
foreach($db->query("SELECT * FROM categories WHERE parent IS NULL") as $parent)
{
$t=0;
categories($db,$t,$parent['id'],$parent['category']);
}
function categories($db,$t,$id,$category)
{
echo"<option value=".$id.">";//
for($i=0;$i<$t;$i++) {echo" "; $GLOBALS['cat'].=" ";}
echo $category."</option>";//" ".$id.
$GLOBALS['cat'].=$category."<br>";
$t++;
if($db->query("SELECT * FROM categories WHERE parent=".$id)->rowCount()>0)
{
foreach($db->query("SELECT * FROM categories WHERE parent=".$id) as $child)
categories($db,$t,$child['id'],$child['category']);
}
}
Do you have a faster solution?
Thanks

Answer 1

If you know the depth of your tree (maximum or desired), you can have it all in SQL, flattened to a row representing path:
SELECT c.category, c1.category AS sub1, c2.category AS sub2, c3.category AS sub3
FROM categories c
LEFT JOIN
(categories c1
LEFT JOIN
(categories c2
LEFT JOIN categories c3
ON c3.parent = c2.category)
ON c2.parent = c1.category)
ON c1.parent = c.category
WHERE c.parent IS NULL;
Having that, it's not a big deal to present it in UI accordingly.

Answer 2

$sql = "SELECT id, name, parent_id,
(SELECT COUNT(*) FROM categories WHERE parent_id = cat.id) as child_count
FROM categories cat";
$result = db_select_all($sql);
function build_menu($pid, $level = 1) {
global $result;
$filtered = array_filter($result, function($elem) use($pid) {
return $elem['parent_id'] == $pid;
});
if(empty($filtered)) return;
foreach($filtered as $item) {
extract($item);
echo "<div>" . str_repeat("---", $level) . $name;
build_menu($id, $level + 1);
echo "</div>" . PHP_EOL;
}
}
$menu_elements = array_filter($result, function($item) {
return $item['parent_id'] == '';
});
foreach($menu_elements as $menu) {
$name = $menu['name'];
$id = $menu['id'];
echo "<div>" . $name;
build_menu($id);
echo "</div>" . PHP_EOL;
}