Prova così:

$query = mysql_query("SELECT
$user_tabella.*,
Count($ordini_tabella.ordine) AS Numero,
Sum($ordini_tabella.ordine) AS Somma
FROM
$user_tabella LEFT JOIN $ordini_tabella
ON
$user_tabella.codice=$ordini_tabella.codcliente
GROUP BY $user_tabella.nome
HAVING Sum($ordini_tabella.ordine)<100");