Null value and analytical functions

SQL> select ename, sal, DENSE_RANK() over (order by sal NULLS FIRST) toprank
  2  from emp
  3  order by sal;

ENAME             SAL    TOPRANK
---------- ---------- ----------
MARTIN           1250          2
WARD             1250          2
ADAMS            1500          3
TURNER           1500          3
CLARK            2850          4
BLAKE            2850          4
JONES            2975          5
KING             3000          6
SCOTT            3000          6
SMITH                          1
ALLEN                          1

11 rows selected.

SQL>
SQL> select ename, sal, DENSE_RANK() over (order by sal NULLS LAST) toprank
  2  from emp
  3  order by sal;

ENAME             SAL    TOPRANK
---------- ---------- ----------
WARD             1250          1
MARTIN           1250          1
TURNER           1500          2
ADAMS            1500          2
BLAKE            2850          3
CLARK            2850          3
JONES            2975          4
SCOTT            3000          5
KING             3000          5
SMITH                          6
ALLEN                          6

11 rows selected.

SQL>