SELECT reparatie.id_client, clienti.nume, COUNT(reparatie.id_client) AS cnt
    FROM reparatie join clienti ON clienti.id = reparatie.id_client WHERE
    reparatie.data >= TO_DATE('01-12-2019', 'DD-MM-YYYY') AND reparatie.data <= TO_DATE('31-12-2019', 'DD-MM-YYYY')
    GROUP BY reparatie.id_client, clienti.nume
    ORDER BY cnt DESC;

UPDATE reparatie SET COST=(cost-0.25*COST) WHERE id=(
    SELECT reparatie.id FROM reparatie r, clienti c WHERE r.id_client=c.id AND c.nume='Ionescu' AND ROWNUM=1 ORDER BY r.id DESC);