Creating an "update" page MYSQL/PHP - php

Question

I'm currently trying to make a page via php which allows the user to update data in my database. I'm experiencing two problems: first when I run my code I get the "Error: Query was empty", however updates were made to the database and this leads me to my second problem. Fields that were left empty (a user doesn't have to enter data into all the fields if they only have one or two things to update) become blank after the updates are made. This is because my current script updates all elements, but is there any way I can have it where if the user leaves an input field blank, nothing gets changed when the database is updated?
Here is my code:
if (isset($_POST['submit'])) {
$id = $_POST['id'];
$lastname = $_POST['lastname'];
$firstname = $_POST['firstname'];
$color = $_POST['color'];
$number = $_POST['number'];
// need id to be filled and need at least one other content type for changes to be made
if (empty($id) || empty($lastname) and empty($firstname) and empty($major) and empty($gpa)) {
echo "<font color='red'>Invalid Submission. Make sure you have an ID and at least one other field filled. </font><br/>";
} else {
// if all the fields are filled (not empty)
// insert data to database
mysql_query ("UPDATE students SET lastname = '$lastname', firstname = '$firstname', favoritecolor = '$color', favoritenumber = '$number' WHERE id = '$id'");
if (!mysql_query($sql,$con)) {
die('Error: ' . mysql_error());
}
// display success message
echo "<font color='blue'>Data updated successfully.</font>";
// Close connection to the database
mysql_close($con);
}
}

Answer 1

To answer your question, you need to catch the query's result and check for errors on that.
$query = mysql_query(/*query*/);
if (!$query)
//error handling
Be sure to read up on SQL injections, as per my comment.

Answer 2

To better help you understand the behavior you were seeing, I will explain to you what was wrong with your code:
mysql_query ("UPDATE students SET lastname = '$lastname', firstname = '$firstname', favoritecolor = '$color', favoritenumber = '$number' WHERE id = '$id'");
That first part was executing a MySQL query, regardless of that fact that you did not assign it's return value to a variable.
if (!mysql_query($sql,$con)) {
die('Error: ' . mysql_error());
}
The second part was attempting to run a query by passing the first parameter $sql which has not been set, and the second parameter $con which also appears to not have been set. The first query you ran executed just fine while the second one could never execute. Your solution:
$result = mysql_query(
"UPDATE students
SET lastname = '$lastname', firstname = '$firstname',
favoritecolor = '$color', favoritenumber = '$number'
WHERE id = '$id'"
);
if (!$result) {
throw new Exception('Error: ' . mysql_error());
// or die() is fine too if that's what you really prefer
}

Answer 3

if (!mysql_query($sql,$con)) Here $sql and $con are not defined. Should you be running mysql_query twice?

Answer 4

Few guesses:
There is no mysql connect function I assume it's called elsewhere
Print out your query string. I've always found explicitly denoting what is a string and what is a variable by 'SELECT * FROM '.%tblvar.';'; to be much more debug friendly.