Islamfon, Ни черта тебя не поняла.

Выводит все записи, без повторений id_user:
$res = mysql_query(\"select DISTINCT id_user from `test` ORDER BY `ball` DESC LIMIT 10;\";

Выводит только записи с id_user = 1
$res = mysql_query(\"select * from `test` WHERE `id_user` = \'1\' ORDER BY `ball` DESC LIMIT 10;\";

$row = mysql_fetch_array(mysql_query(\"SELECT * FROM `test` ORDER BY `ball` DESC LIMIT 1\");