สอบถามครับการใช้ session_start เพื่อกันไว้ไม่ให้ผู้ที่ไม่ได้ login ใช้งานได้

if ($_POST[button] == 'ตกลง'){

                    $username = $_POST ['username'];

                    $sqlStudent     = "SELECT * FROM student WHERE username='$_POST[username]' AND password='$_POST[password]'";
                    $queryStudent   = mysql_query($sqlStudent)or die(mysql_error());
                    $student        = mysql_num_rows($queryStudent);

                    $sqlPersonnel   = "SELECT * FROM personnel WHERE username='$_POST[username]' AND password='$_POST[password]'";
                    $queryPersonnel = mysql_query($sqlPersonnel)or die(mysql_error());
                    $personnel      = mysql_fetch_array($queryPersonnel);

                        if($student == 1){
                            $_SESSION['username'] = $username;
                            echo "<script>window.location='mainStudent.php';</script>";
                        }else{
                            if($personnel[cyp_id] == 1){
                                $_SESSION['username'] = $username;
                                echo "<script>window.location='mainTeacher.php';</script>";
                            }else{
                                if($personnel[cyp_id] == 2){
                                    $_SESSION['username'] = $username;
                                    echo "<script>window.location='mainOrganization.php';</script>";
                                }else{
                                    if($personnel[cyp_id] == 3){
                                        $_SESSION['username'] = $username;
                                        echo "<script>window.location='mainAdmin.php';</script>";
                                    }else{
                                        echo "<table>";
                                        echo "<tr>";
                                        echo "<td align=\"center\" width=\"250\" bgcolor=\"orangered\">";
                                        echo "โปรดตรวจสอบชื่อผู้ใช้หรือรหัสผ่าน";
                                        echo "</td>";
                                        echo "</tr>";
                                        echo "</table>";
                                    }
                                }
                            }
                        }
                    }