如何将两列值相乘并将其写入另一列？

I populated mysql database to html table and now what I want to do is multiply price and quantity in order to find total in order to show in another column. These are all about one table.

"SELECT * FROM  orders where user_id = '".$_SESSION['userId']."' ORDER BY order_id DESC";

This is my sql statement. And this is my related php code :

                    foreach( $result as $data )
                        {

                            echo "<tr><td>".$data['order_situation']."</td>
                                      <td>".$data['client_name']."</td>
                                      <td>".$data['client_last_name']."</td>
                                      <td>".$data['city']."</td>
                                      <td>".$data['district']."</td>
                                      <td>".$data['area']."</td>
                                      <td>".$data['address']."</td>
                                      <td>".$data['tel_no1']."</td>
                                      <td>".$data['tel_no2']."</td>
                                      <td>".$data['product']."</td>
                                      <td>".$data['quantity']."</td>
                                      <td>".$data['price']."</td>
                                      <td>".$data['down_payment']."</td>
                                      <td>".$data['explanation']."</td>
                                      <td>".$data['username']."</td>
                                      <td>".$data['date']."</td>
                                      <td><a href=\"updateorders.php?id=".$data['order_id']."\">Güncelle</a></td>
                                   </tr>";

                        }

"SELECT *, price * quantity AS total FROM orders WHERE user_id = '".$_SESSION['userId']."' ORDER BY order_id DESC";

That should work. Then you can reference the total: $data['total']