检查mysql数据库中是否存在用户名？

Question

Ok so have this code, its for a registration form but i can't get it to check if there is already the username taken? all it does is creates the same username in the database again, instead of returning "Username already taken" if anyone could help would be very appreciated.

$checkusernametaken = mysql_query("SELECT FROM users (username) WHERE username=username");
 if(mysql_num_rows($checkusernametaken)>=1)
    {
        echo"Username already exists";
    }
 else
    {
        $sql = "INSERT INTO users(username, password) VALUES(:username, :password)";
        $stmt = $con->prepare( $sql );
        $stmt->bindValue( "username", $this->username, PDO::PARAM_STR );
        $stmt->bindValue( "password", hash("sha256", $this->password . $this->salt), PDO::PARAM_STR );
        $stmt->execute();
        return "Registration Successful Welcome " + $checkusernametaken + "<br/> <a href='index.php'>Login Now</a>";
    }

This is the code i used and i got it working with the help of everyone in the comments, thanks heaps guys!

try {
        $con = new PDO( DB_DSN, DB_USERNAME, DB_PASSWORD );
        $con->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
        $sql = "INSERT INTO users(username, password) VALUES(:username, :password)";

        $stmt = $con->prepare( $sql );
        $stmt->bindValue( "username", $this->username, PDO::PARAM_STR );
        $stmt->bindValue( "password", hash("sha256", $this->password . $this->salt), PDO::PARAM_STR );
        $stmt->execute();
        return "Registration Successful <br/> <a href='index.php'>Login Now</a>";
    }
    catch( PDOException $e ) 
    {
         return "Username Taken <br/> <a href='register.php'>Try Again?</a>";
    }

Answer 1

Here is an example of how the table would look like:

CREATE TABLE `users` (
`id` INT AUTO_INCREMENT PRIMARY KEY NOT NULL,
`username` VARCHAR(30) NOT NULL UNIQUE,
`password` VARCHAR(64) NOT NULL
);

Live DEMO.

The simplest way would be if you change your table to make the field username unique, this way if you try to insert the same username it will result on a duplicate entry error which you can easily catch like this:

try
{
    $sql = "INSERT INTO users(username, password) VALUES(:username, :password)";
    $stmt = $con->prepare($sql);
    $stmt->bindValue("username", $this->username, PDO::PARAM_STR);
    $stmt->bindValue("password", hash("sha256", $this->password . $this->salt), PDO::PARAM_STR);
    $stmt->execute();
    return "Registration Successful Welcome {$checkusernametaken}<br/><a href=\"index.php\">Login Now</a>";
}
catch (PDOException $e)
{
    if ($e->getCode() == 1062)
    {
        return "The selected username is not available...";
    }
    else
    {
        // More info at: http://php.net/manual/en/class.pdoexception.php
        return "Handle any other errors here.";
    }
}

Answer 2

$checkusernametaken = mysql_query("SELECT FROM users (username) WHERE username=username");

在这里你应该改为

$checkusernametaken = mysql_query("SELECT * FROM `users` WHERE lcase(`username`) = lcase('".mysql_escape_string($this->username)."')") or die(mysql_error());