Why am I getting this error "Error: Query was empty"

I am trying to update my SQL database using a form through php, but i keep getting the error "Error: Query was empty".

<?php
$sql = "";
$con = mysql_connect("*******","*******");
if (!$con)
{
die('Could not connect: ' . mysql_error());
}

mysql_select_db("*******", $con);
mysql_query($sql, $con);


if (isset($_POST['STUDENT_FNAME'], $_POST['STUDENT_SNAME'],
$_POST['STUDENTNO'] ))
{ 

$sql="UPDATE STUDENT SET STUDENT_FNAME=('$_POST[STUDENT_FNAME]'),
STUDENT_SNAME=('$_POST[STUDENT_SNAME]')
WHERE STUDENTNO=
('$_POST[STUDENTNO]')";
}

if (!mysql_query($sql,$con))
{
die('Error: ' . mysql_error());
}
echo "1 record updated";

mysql_close($con);
?>

It also won't update my table and I don't know what I've done wrong. All help will be much appreciated. I am new to this as you can probably tell!