SELECT DISTINCT firm.name
FROM firm, invoce, invoce AS i2
WHERE 
    date BETWEEN '2006-09-01' AND '2006-09-30' AND
    invoce.bank = firm.bank AND
    i2.bank = firm.bank AND i2.id != invoce.id AND
    (SELECT COUNT(*)
FROM pos AS p2, pos AS p1
WHERE 
p1.inv_id = invoce.id AND
p2.inv_id != pos.id AND
p1.code = p2.code AND
p1.id != p2.id
    ) = 0
;