ขอรบกวนหน่อยค่ะเราจะเอาค่าตัวแปร $sum ไปอัพเดทลงฐานข้อมูลยังไงอ่ะคะ

<td align="center"><? echo $row['id_prd']?></td>
<? $id=$row['id_prd'];?>
<td align="center"><? echo $row['name_prd']?></td>
<td align="center"><?php echo $row['in_stock_prd']; ?></td>
<td align="center"><?php echo $row['order_num']; ?></td>
<? $sum=$row['in_stock_prd']-$row['order_num'];?>
<td align="center"><?php echo $sum; ?></td>
</tr>
<?php $i++; } ?>
</table>
<p>&nbsp; </p>
         </td>
      </tr>
    </table>
    <?
	$query2 = "UPDATE product SET in_stock_prd='".$sum."' WHERE id_prd='".$id."'" ;
									  
	mysql_query($query2) or die(mysql_error()); 
	?>

<?	
		 	$query = mysql_query("
				SELECT product.id_prd,sum(order_detail.number) as order_num,in_stock_prd,name_prd
				FROM order_detail,product
				WHERE product.id_prd=order_detail.ref_id_prd
				GROUP BY ref_id_prd
				ORDER BY order_num DESC");
				$i=1;
		 ?>
         <p align="center"><center><h3>ยอดสินค้าทั้งหมด</center></h3></p>
<table align="center" border="1" cellpadding="8">
<tr bgcolor="#00FF00">
<td align="center"><div align="center">ลำดับ</div></td>
<td align="center"><div align="center">รายการสินค้า</div></td>
<td align="center"><div align="center">ยอดทั้งหมด</div></td>
<td align="center"><div align="center">ขายแล้ว</div></td>
<td align="center"><div align="center">คงเหลือในคลังทั้งหมด</div></td>
</tr>
<?php while ($row = mysql_fetch_array($query)) { ?>
<tr>
<td align="center"><? echo $row['id_prd']?></td>
<? $id=$row['id_prd'];?>
<td align="center"><? echo $row['name_prd']?></td>
<td align="center"><?php echo $row['in_stock_prd']; ?></td>
<td align="center"><?php echo $row['order_num']; ?></td>
<? $sum=$row['in_stock_prd']-$row['order_num'];?>
<td align="center"><?php echo $sum; ?></td>
</tr>
<?php $i++; } ?>
</table>
<p>&nbsp; </p>
         </td>
      </tr>
    </table>
    <?
	$query2 = "UPDATE product SET in_stock_prd='".$sum."' WHERE id_prd='".$id."'" ;
									  
	mysql_query($query2) or die(mysql_error()); 
	?>

echo $query2;