All stock saved already in stock table. But how can I update stock quantity when I sell my product? I want to update my stock table quantity row. I'm using MySQL.

table name `stock`:

```
--------------------------------------------------------------------
    id|username | date      | item     | quantity|  amount
------------------------------------------------------------------
    1 |xyz      |2013-10-09 | computer |25       | 25.00
-----------------------------------------------------------------

```

and this is `sale` table:

```
--------------------------------------------------------------------
    id|username | date      | item     | quantity|  amount
------------------------------------------------------------------
    1 |xyz      |2013-10-09 | computer |25       | 25.00
-----------------------------------------------------------------

```

and this is sale.php page. When I sell a product this page save a record in sale table but i want to update stock table quantity row:

```
 <?php
/* 
 NEW.PHP
 Allows user to create a new entry in the database
*/

 // creates the new record form
 // since this form is used multiple times in this file, I have made it a function that is easily reusable
 function renderForm($date ,$username,$item,$quantity,$amount, $error)

 {
 ?>
 <form id="searchform" action="" method="post" enctype="multipart/form-data">
  <div align="center">
 <fieldset>

   <div align="center">
     <legend align="center" >Stock!</legend>
   </div>
   <div class="fieldset">
     <p>
   <label class="field" for="date">Date: </label>

       <input name="date" type="text" class="tcal" value="<?php echo date("Y-m-d");; ?>" size="30"/>
         </p>
   <p>
     <label class="field" for="username">User Name : </label>

     <input name="username" type="text"  id="username"  value="<?php echo $username; ?>" size="30"/>
   </p>
   <p>
     <label class="field" for="item">Item: </label>

     <input name="item" type="text"  value="<?php echo $item; ?>" size="30"/>
   </p>

       <p>
       <label class="field" >Quantity :</label>
       <input  name="quantity" type="text" value="<?php echo $quantity; ?>"  size="30"/>
     </p> 
       <p>
       <label class="field" >Amount :</label>
       <input  name="amount" type="text" value="<?php echo $amount; ?>"  size="30"/>
     </p> 
  </div>
 </fieldset>
   <p align="center" class="required style3">Please Fill The Complete Form </p>
   <div align="center">
     <input name="submit" type="submit" class="style1" value="Submit">

   </div>
 </form> 

 <?php 
 // if there are any errors, display them
 if ($error != '')
 {
 echo '<div style="padding:4px; border:1px solid red; color:red;">'.$error.'</div>';
 }
 ?> 

 <?php 
 }

 // connect to the database
 include('connect-db.php');

 // check if the form has been submitted. If it has, start to process the form and save it to the database
 if (isset($_POST['submit']))
 { 
 // get form data, making sure it is valid
 $date = mysql_real_escape_string(htmlspecialchars($_POST['date']));
 $username = mysql_real_escape_string(htmlspecialchars($_POST['username']));
 $item = mysql_real_escape_string(htmlspecialchars($_POST['item']));
 $quantity = mysql_real_escape_string(htmlspecialchars($_POST['quantity']));
 $amount = mysql_real_escape_string(htmlspecialchars($_POST['amount']));

 // check to make sure both fields are entered
 if ($date == '' || $quantity == '')
 {
 // generate error message
 $error = 'ERROR: Please fill in all required fields!';

 // if either field is blank, display the form again
 renderForm($date ,$username,$item,$quantity,$amount,  $error);

 }
 else
 {
 // save the data to the database
  mysql_query("INSERT  sale SET date='$date', username='$username',item='$item',quantity='$quantity',amount='$amount'")
 or die(mysql_error()); 
 echo "<center>Stock Enter Complete!</center>";
 // once saved, redirect back to the view page

 }
 }
 else
 // if the form hasn't been submitted, display the form
 {
 renderForm('','','','','','','','','');
 }

         ?>

```