tip : dbms_random

SELECT dbms_random.string('U', 2)||TRUNC(dbms_random.VALUE(1000, 9999))
FROM dual;

Output : au3910

SELECT dbms_random.string('U', 2)||TRUNC(dbms_random.VALUE(1, 9))||
dbms_random.string('U', 2)||TRUNC(dbms_random.VALUE(1,9))||
dbms_random.string('U', 2)
FROM dual;

Output : AI7KU4EE