Only update entry when there is a new value - php

Question

I have a form that will update specified entries in a mysql table. The form will only submit if all the fields are filled in .
Is there a way to make it so that the form will only update fields that have a new value and leave the ones that have been left blank?
Form code :
<?php
error_reporting(E_ALL ^ E_DEPRECATED);
if(isset($_POST['update']))
{
$con = mysql_connect($server, $db_user, $db_pass);
if(! $con )
{
die('Could not connect: ' . mysql_error());
}
$id = $_POST['id'];
$english = $_POST['english'];
$math = $_POST['math'];
$science = $_POST['science'];
$table = $_POST['year'];
$sql = "UPDATE $table ".
"SET english = $english ,math = $math ,science = $science ".
"WHERE id = $id" ;
mysql_select_db('education');
$retval = mysql_query( $sql, $con );
if(! $retval )
{
die('Could not enter data: ' . mysql_error());
}
//header("Location: " . $_SERVER['PHP_SELF']);
//echo "Entered data successfully\n";
mysql_close($con);
}
else
{
?>
<?php } ?>
<h4 align="center">Update student details</h4>
<form action="<?php $_SERVER['PHP_SELF'] ?>" method="post">
Student ID:
<input name="id" type="text" id="id">
<br>
English mark:
<input name="english" type="number" id="english">
<br>
Math's mark:
<input name="math" type="number" id="math">
<br>
Science mark:
<input name="science" type="number" id="science">
<br>
Year: <br>
<select name="year" id="year">
<option value="">Select...</option>
<option value="year1">Year 1</option>
<option value="year2">Year 2</option>
<option value="year3">Year 3</option>
<option value="year4">Year 4</option>
</select>
<br>
<br>
<input name="update" type="submit" id="update" value="Submit">
</form>

Answer 1

use isset() before use the variables in your query

Answer 2

try this :
$sql = "UPDATE $table ".
"SET english = $english ,math = $math ,science = $science ".
"WHERE id = $id AND (english != $english OR math != $math OR science != $science)";