SQL> with test as
  2  (select 'A' let, 'Roma' nam from dual union all
  3   select 'A', 'Los Angeles'  from dual union all
  4   select 'B', 'New york'     from dual union all
  5   select 'C', 'Rio'          from dual union all
  6   select 'C', 'London'       from dual union all
  7   select 'C', 'Budapest'     from dual
  8   )
  9  select
 10    let,
 11    nam,
 12    rank() over (order by let) rnk,
 13    dense_rank() over (order by let) drnk
 14  from test
 15  order by let, nam;

L NAM                RNK       DRNK
- ----------- ---------- ----------
A Los Angeles          1          1
A Roma                 1          1
B New york             3          2
C Budapest             4          3
C London               4          3
C Rio                  4          3

6 rows selected.

SQL>