Lesley Peters Lesley Peters - 3 months ago 27x
SQL Question

PHP exception handling after SQL insert

I am creating a very simple registration form in php, currently when the user tries to register there will popup a javascript alert with a succes or fail message.

Now I want to catch the sql exception to show if the username or email already excists in the database instead of a standard fail message.

This is the code I have so far:

$uname = mysql_real_escape_string($_POST['uname']);
$email = mysql_real_escape_string($_POST['email']);
$upass = md5(mysql_real_escape_string($_POST['pass']));

if(mysql_query("INSERT INTO user(username,password,email) VALUES('$uname','$upass','$email')"))
<script>alert('successfully registered ');</script>
<script>alert('error while registering you...');</script>

How can I check if the email or username already excists in the database? Both variable's are already unique in the database.


From Comments:

I don't want 2 queries while the database can return an exception for me. If there are about 10 million records in that table, I don't want to check them all before inserting a new one.

Ok, so you have one query to insert and check is unique? So you have to INSERT on a UNIQUE_INDEX MySQL column, you can catch these sort of exceptions with the following style of answer shameless stolen from this answer to this question:

In the case of this answer we'll assume you're using PDO, because you should. Please read up about it.

// Pre-setup the database connection somewhere, usually an include (or a class)
$link = new PDO("mysql:host=$dbhost;dbname=$dbname",$dbusername,$dbpassword);
// PDO needs to be set in Exception mode:

//Input Processing functions.
// (entirely optional)
$uname = MyCleanerFunction($_POST['uname']);
$email = MyCleanerFunction($_POST['email']);
//please see note below re:MD5
//$upass = md5($_POST['pass']); 
$options['cost'] = 12;
$upass = password_hash($_POST['pass'],PASSWORD_BCRYPT,$options);

//now reach the code part:
    try {
        //PDO query execution goes here:

         $statement = $link->prepare("INSERT INTO user(username,password,email) VALUES(:uname, :email, :pass)"));
         $statement->bindValue(":uname", $uname);
         $statement->bindValue(":email", $email);
         $statement->bindValue(":pass", $upass);
         //reaching here everything is ok! 
    catch (\PDOException $e) {
        if ($e->errorInfo[1] == 1062) {
            // The INSERT query failed due to a key constraint violation.
            // THIS means that it failed because the Primary Key 
            // (the email) appears already in the database table.
        if($e->errorInfo[1] == 9999){
          // possible other IF clauses for other errors in INSERT.

You would also do well to read up about catching and outputting PDO errors. As well as all about MySQL Unique Key Constraints.

  • Also very useful alternative viewpoint that you Should not catch PDO exceptions.

  • Also please note that MD5 is an extremely weak hash for storing passwords and that PHP password_hash function is the much preferred way of doing it.

  • PLEASE use Prepared Statements for your MySQL interactions, the layout above is a rough guide to how they look and is very similar for MySQLi and PDO. Prepared Statements go a long way towards securing your data from malicious user input.