Retrieve Category & Subcategory list in PHP - php

Question

I'm very new to HTML, PHP & MySql, so apologies for the noob question but, I need to retrieve categories from a database, that are in 2 tables, but am having difficulty calling the subcategories after clicking the initial link.
//Category code as...
<?php
require 'dbcon.php';
$sql = "SELECT * FROM `category`";
$result = mysqli_query($conn, $sql) or die(mysqli_error($conn));
echo "<table>
< tr >
<th>Category < /th>
< /tr>";
while ($row = $result ->fetch_assoc()) {
$id = $row['category_ID'];
echo "<td>" .$row['description']. "</td>";
echo "</tr>";
}
echo "</table>";
?>
//Subcategory code as...
<?php
require 'database_conn.php';
$sql = "SELECT category.ID, subcategory.title, subcategory.ID FROM category, subcategory WHERE subcategory.ID=category.ID";
$result = mysqli_query($conn, $sql) or die(mysqli_error($conn));
echo "<table>
< tr >
<th>Title < /th>
< /tr>";
while ($row = mysqli_fetch_array($result)) {
echo "<tr>";
echo "<td>" .$row['title']. "</td>";
echo "</tr>";
}
echo "</table>";
mysqli_close($conn);
?>
Ideally, I would like it so that selecting one category will bring up the whole list of subcategories, either in the same page or a separate page. I've struggled over this for a few days now and I'm really struggling to understand what it is I am doing wrong.

Answer 1

simply take the id from the URL and put into your query
$category_id = $_GET['id'];
SELECT * FROM subcategory WHERE category_id = $id;