ผมอยากให้มันตัดสต๊อคโดย update(ตัดสต๊อค) จากที่มีอยู่และก็ insert ข้อมูลไปที่ table ใหม่

<?php

include_once '../dbconfig.php';
$message = "แก้ไขข้อมูลเรียบร้อยแล้ว !!!";

$id = $_REQUEST['id'];
$piece = $_POST['piece'];
$productname = $_POST['productname'];
$pieceexport = $_POST['pieceexport'];
$factory = $_POST['factory'];
$newpiece = $_POST['piece'] - $_POST['pieceexport'];

if (!isset($errMSG)) {

    $sql = " INSERT INTO exportorder
 					(productname,pieceexport,factory)
 					VALUES
 					('$productname','$pieceexport','$factory')";
    $sql2 = " UPDATE product1 SET

 			piece = '$newpiece'
  			WHERE id = $id ";

    if (mysqli_query($mysqli, $sql) && mysqli_query($mysqli, $sql2))
    {
        echo "<script type='text/javascript'>alert('$message');</script>";
        header("refresh:1;viewp1.php");
    } else
        {
        echo "Error: " . $sql . "<br>" . mysqli_error($mysqli);
    }
}
?>