Mysqli select multiple queries - php

Question

I'm trying to select one query by using the user_id to get the categories specific to the user and then from there use the results from the first query to select data from a second table in a second query. The data from the second query I would like to display in a drop down menu. Is there any easy way to do this? I'm new to using mysqli so please bear with me.
I have the following function but it's not displaying anything in the dropdown menu, it is showing the drop down menu though.
function get_classes($mysqli) {
if(isset($_SESSION['user_id'], $_SESSION['username'], $_SESSION['login_string'])) {
$user_id = $_SESSION['user_id'];
if ($stmt = $mysqli->prepare("SELECT category_id
FROM questions WHERE user_id = ?")) {
$stmt->bind_param('i', $user_id);
$stmt->execute();
$stmt->store_result();
if($stmt->num_rows>0) {
$stmt->bind_result($category_id);
$stmt->fetch();
if($result = $mysqli->prepare("SELECT category_id, category_name
FROM category WHERE category_id = ?")) {
$result->bind_param('s', $category_id);
$result->execute();
$result->store_result();
}
}
echo "<select id='classes' name='classes'>";
while ($row = $result->fetch_assoc()) {
unset($user_id, $category_name);
$category_id = $row['category_id'];
$category_name = $row['category_name'];
echo '<option value="'.$category_id.'">'.$category_name.'</option>';
}
echo "</select>";
}
}
}
I'm calling the function from another page with the following code:
<?php
get_classes($mysqli);
?>

Answer 1

You can make all via one sql query:
SELECT c.category_id, c.category_name
FROM questions q
INNER JOIN category c ON c.category_id = q.category_id
WHERE q.user_id = ?

Answer 2

Alternatively, you could join them instead, then the normal fetching. Example:
function get_classes($mysqli) {
if(!isset($_SESSION['user_id'], $_SESSION['username'], $_SESSION['login_string'])) {
return false;
}
$user_id = $_SESSION['user_id'];
$sql = '
SELECT
category.category_id,
category.category_name
FROM category
JOIN questions
ON questions.category_id = category.category_id
WHERE questions.user_id = ?
';
$stmt = $mysqli->prepare($sql);
$stmt->bind_param('i', $user_id);
$stmt->execute();
$result = $stmt->bind_result($category_id, $category_name);
// printing
echo '<select name="classes" name="classes">';
while($row = $stmt->fetch()) {
echo '<option value="'.$category_id.'">'.$category_name.'</option>';
}
echo '</select>';
}