为什么我得到MySQL错误“查询为空”？

$id = $_REQUEST['id'];
$Section = $_REQUEST['section'];
$Subject = $_REQUEST['subject'];
$type = $_REQUEST['type'];
$Start_date1 = isset($_REQUEST['startTxt'])?($_REQUEST['startTxt']):"";
$Venue = isset($_REQUEST['venTxt'])?($_REQUEST['venTxt']):"";
$Facilitator = isset($_REQUEST['faciTxt'])?($_REQUEST['faciTxt']):"";
$Level = isset($_REQUEST['lvlLst'])?($_REQUEST['lvlLst']):"";
$Date1 = $_REQUEST['date1'];

if(isset($_REQUEST['EDIT']))
{
    mysql_query("UPDATE service SET Start_date='$Date1', Venue='$Venue', Facilitator='$Faci' WHERE ServiceID ='$id'");
    if (!mysql_query($sql,$con))
    {
        die('Error: ' . mysql_error());
    }

    echo '<script type="text/javascript">';
    echo 'alert("Changes have been save!");';
    echo 'window.location="Admin_RecSchedMapLst.php";';
    echo '</script>';
    mysql_close($con);
}           
$id = $_REQUEST['id'];
$Section = $_REQUEST['se