Can't get to show all of a tree in a traversal preorder in php

Question

I have a table in my database that host many genealogy trees.

-----------------------------
- id  name              parent_id
-----------------------------
- 1   grandfather       NULL
- 2   father            1
- 3   uncle             1
- 4   son               2
- 5   brother           2
- 6   cousin's dauther  7
- 7   cousin            8
- 8   auntie            1

The problem is I can't get to show all the names because of an edge case:

-When I have a person with a parent_id that is bigger than it's parent's parent_id (see cousin's daugter)

I use this queries to get the table:

    $sql = "SELECT p1.id, p1.name, p1.parent_id FROM pariente p1 
    ORDER BY p1.parent_id";
    $result = $conn->query($sql);

The problem is that if I use "ORDER BY parent_id" "cousin's dauther" won't show and if I use "ORDER BY id" "cousin" won't show.

I use this functions to make a the tree into an array and draw it:

        function make_tree($data, $root) {
            $tree = [];
            foreach ($data as $node) {
                insert($tree, $node);
            }

            return $tree;
        }

        function insert(&$root, &$node) {
            if (!$root) {
                $root = $node;
            }
            else if ($root["id"] === $node["parent_id"]) {
                $root["children"][] = $node;
            }
            else if (array_key_exists("children", $root)) {
                foreach ($root["children"] as &$c) {
                    if (insert($c, $node)) {
                        break;
                    }
                }
            }
        }

        function preorder2(&$root) {
            if ($root) {
                echo "<li>";
                echo $root["name"];

                if (array_key_exists("children", $root)) {
                    echo "<ul>";
                    foreach ($root["children"] as $c) {
                        preorder2($c);
                    }
                    echo "</ul>";
                }
                echo "</li>";
            }
        }
    ?>

And after I use this to call the functions:

<div>

<?php
while( $row = mysqli_fetch_assoc( $result)){
    $resguard[] = $row;
}
    $tree = make_tree($resguard);
    preorder2($tree);
?>
</div>

Answer 1

I had a similar problem once, and here's how I fixed it.

1. Iterate over the dataset, putting each node in your array, and keep track of what you want to be your root node.

2. Iterate over the array. For each node where the parent_id is not null, lookup the parent node by id, and add the current node as a child. There's no need to use recursion when building the tree.

Answer 2

At the end I believe I didn't get toastifier answer, but it get me thinking and I ended up solving the problem (still using the way ggorlen shows the tree).

First this are the queries:

$sql = "SELECT p1.id, p2.name, p2.id as minor, p2.name FROM pariente p1 INNER JOIN pariente p2 ON p1.id = p2.parent_id ORDER BY p1.id";
$result = $conn->query($sql);

$sql2 = "SELECT p1.id, p1.nombre, p1.padre_id FROM pariente p1 WHERE p1.padre_id IS NULL ORDER BY p1.id";
$raices = $conn->query($sql2);

The functions:

        function make_tree($resguardo, $root){
            $tree = [];
            $tree = $root;
            foreach ($resguardo[$root["id"]] as $node) {
                add($tree, $node, $resguardo);
            }
            return $tree;
        }

        function add(&$root, &$children, $resguardo){
            $root["children"][$children["minor"]] = $children;

            $flag= false;
            if (isset($resguardo[$children["minor"]])) {
                $flag = true;
            }

            if ($flag == false){
                return;
            } else {
                foreach ($resguardo[$children["minor"]] as $child) {
                    agregar($root["children"][$children["minor"]], $child, $resguardo);
                }
            }
        }

        function preorder2(&$root) {
            if ($root) {
                echo "<li>";
                echo '<a href="">';
                echo $root["name"];
                echo "</a>";

                if (array_key_exists("children", $root)) {
                    echo "<ul>";
                    foreach ($root["children"] as $c) {
                        preorder2($c);
                    }
                    echo "</ul>";
                }
                echo "</li>";
            }
        }

And I call them here:

while( $row = mysqli_fetch_assoc($result)){
    $resguardo[$row["id"]][] = $row;
}
    while( $root = mysqli_fetch_assoc( $roots)){
    echo '<ul>';
    $tree = make_tree($resguardo, $root);
    preorder2($tree);
    echo "</ul>";
} 

Thanks alot to both of you, I would never solve it without your help.