$data = $db->query("SELECT * FROM users");

foreach ($data as $row) {
   $data2 = $db->one("SELECT amount FROM cart WHERE id = " . $row['id']);
   echo $row['name'] . '<br>';
   echo $data2['amount'] . '<br>';

}

$data = $db->query("SELECT *"
  . "n (SELECT SUM(amount) FROM cart WHERE user.id = cart.user_id) as total"
  . "n FROM users";