Hocam su sekilde yapin eger girilen veri email adressi ise ayri, düz yazi ise ayri sorgu yaptirin. Yani su sekilde:

if(filter_var($email, FILTER_VALIDATE_EMAIL)){
$sql_addition = "email = '{$email}'";
} else {
$sql_addition = "username = '{$email}'";
}

$query = mysql_query("select * from uyeler where {$sql_addition} and parola = {$pass}");