ถามเรื่อง ตัดสต็อกสินค้า หน่อยครับ 1 ใบเสร็จ (order) ของผมสั่งของได้หลายอย่าง

<?
$strSQL = "SELECT * FROM orderdetails WHERE ORDER_ID = '$OrderID'";
$objQuery = mysql_query($strSQL);
while($objResult = mysql_fetch_array($objQuery))
{
	$strSQL = "UPDATE product SET STOCK = STOCK - '$objResult[Num]' WHERE PRODUCT_ID = '$objResult[PRODUCT_ID]'";
	mysql_query($strSQL);
}
?>

public void ManageStoge(string OrderID,string conStr )
{
 SqlConnection con = new SqlConnection();
 con.ConnectionString = conStr;
 con.Open();
 string strCom = "SELECT * FROM orderdetails WHERE ORDER_ID = '"+OrderID+"'";
 SqlCommand command = new SqlCommand(strCom,con);
 SqlDataReader reader = command.ExecuteReader();
 while(reader.Read())
 {
   string strCom = "UPDATE product SET STOCK = STOCK - '"+reader["Num"].ToString()+"' WHERE PRODUCT_ID = '"+reader["PRODUCT_ID"].ToString()+"'";
 SqlCommand command = new SqlCommand(strCom,con);
 command.ExecuteNonQuery();
 }
 con.Close();
}