我有下面的代码来检查 username 已经用过了，我还需要用4个字段。

if ($valid) {
    $pdo = Database::connect();
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $sql2 = "SELECT * FROM users WHERE username = ?";
    $q2 = $pdo->prepare($sql2);
    $q2->execute(array($username));
    $result = $q2->fetchAll();

    if (count($result) > 0) {
        $usernameError = 'O utilizador já existe!';
    } 
    else{
        $sql = "INSERT INTO users (username,password,role) values(?, ?, ?)";
        $q = $pdo->prepare($sql); 
        $q->execute(array($username,$password,$role)); 
        header("Location: index.php");
    }

    Database::disconnect();
}