สอบถามเรื่อง SELECT * FROM Where และ session ติดปัญหานานมากครับ

<?php
session_start();


if(isset($_SESSION['id'])!="") {
  header("Location: system/index.php");
}

include_once 'config/dbconnect.php';

//check if form is submitted
if (isset($_POST['login'])) {

  $login_user = mysqli_real_escape_string($MySQLi_CON, $_POST['email']);

  $pw_user = mysqli_real_escape_string($MySQLi_CON, $_POST['password']);
  $result = mysqli_query($MySQLi_CON, "SELECT * FROM user_rent WHERE email = '" . $login_user . "' and password = '" . $pw_user . "'");

  if ($row = mysqli_fetch_array($result)) {

     $_SESSION['id_user'] = $row['id_user'];
  $_SESSION['id'] = $row['id'];
  $_SESSION['firstname'] = $row['firstname'];
    $_SESSION['email'] = $row['email'];
    header("Location: system/index.php");
  } else {
    $errormsg = '<div class="alert alert-danger" role="alert"><strong>ล็อกอินไม่ถูกต้อง</strong> หรือบัญชีของท่านยังไม่ได้รับการยืนยัน โปรดลองใหม่อีกครั้ง</div>            <div class="modal-title f-xxl" id="loginModalLabel">ลงชื่อเข้าใช้งานระบบ</div>';
  }
}
?>

 <thead>
                                <tr>
                                    <th>ที่</th>
                                    <th>รหัสการจอง</th>
                                    <th>วันที่จอง</th>
                                    <th>เที่ยวไป</th>
                                    <th>เที่ยวกลับ</th>
                                    <th>ราคารวม</th>
                                    <th>สถานะ</th>
                                    
                                </tr>
                            </thead>

                            <tbody>
  <?php
$res = $mysqli->query("SELECT * FROM ven_rent WHERE id = '".$_SESSION['id']."' ");
while ($row = $res->fetch_assoc()):
?>
<tr>
                      <td><?php echo $row['id_van']?></td>
                      <td><?php echo $row['van_dstart']?></td>
                      <td><?php echo $row['rent_estart']?></td>
                      <td><?php echo $row['rent_address']?></td>             
                      <td><?php echo $row['rent_tel']?></td>
                      <td><?php echo $row['first_rentname']?></td>
                      <td><?php echo $row['first_rentname']?></td>
  
                      <td>
                      <ul class="icons-list">

                      <li><a href="update.php?u=<?php echo $row['id_van'] ?>" data-popup="tooltip" title="แก้ไข"><i class="icon-pencil7"></i></a></li>

<li><a onclick="return confirm('Are you want deleting data')" href="delete.php?d=<?php echo $row['id_van'] ?>"><i class="icon-trash"></i></a></li>
                        
                        <li><a href="#" data-popup="tooltip" title="Options"><i class="icon-cog7"></i></a></li>
                      </ul>
                          </td>
                            <?php
endwhile;
?>
                                                     </tbody>
                        </table>

<?php


session_start();
//PUT THIS HEADER ON TOP OF EACH UNIQUE PAGE
if(!isset($_SESSION['id'])){
  header("location:login.php");
  header("Content-type: text/html; charset=utf-8");
}


include "config.php";


include_once('config/dbconnect.php');

if(isset($_GET['u'])):
     if(isset($_POST['bts'])):
          $stmt = $mysqli->prepare("UPDATE ven_rent SET van_type =?, van_dstart=?, rent_estart=?, rent_address=?, rent_tel=?, first_rentname=? , last_rentname=? ,   rent_amount=?  WHERE id_van=?");
          $stmt->bind_param('ssssssssss', $vr, $vp, $vs, $ve, $rd, $rt, $fr,$lr,$ra,$id);
      
          $vr = $_POST['vr'];
          $vp = $_POST['vp'];
          $vs = $_POST['vs'];
          $ve = $_POST['ve'];
          $rd = $_POST['rd'];
          $rt = $_POST['rt'];
          $rt = $_POST['rt'];
          $fr = $_POST['fr'];
          $lr = $_POST['lr'];
          $ra = $_POST['ra'];
          $id = $_POST['id'];
         
         
          if($stmt->execute()):
        
               echo "<script>location.href='datatable.php'</script>";
          else:
               echo "<script>alert('".$stmt->error."')</script>";
          endif;
     endif;

     $res = $mysqli->query("SELECT * FROM ven_rent WHERE id_van = ".$_GET['u']);
     $row = $res->fetch_assoc();

?>