NULL and row_number

CREATE TABLE EMP (EMPNO NUMBER(4) NOT NULL,
                  ENAME VARCHAR2(10),
                  JOB VARCHAR2(9),
                  SAL NUMBER(7, 2),
                  DEPTNO NUMBER(2));

INSERT INTO EMP VALUES (1, 'SMITH', 'CLERK',    NULL,    20);
INSERT INTO EMP VALUES (2, 'ALLEN', 'SALESMAN', NULL,    30);
INSERT INTO EMP VALUES (3, 'WARD',  'SALESMAN', 1250,    30);
INSERT INTO EMP VALUES (4, 'JONES', 'MANAGER',  2975,    20);
INSERT INTO EMP VALUES (5, 'MARTIN','SALESMAN', 1250,    30);
INSERT INTO EMP VALUES (6, 'BLAKE', 'MANAGER',  2850,    30);
INSERT INTO EMP VALUES (7, 'CLARK', 'MANAGER',  2850,    10);
INSERT INTO EMP VALUES (8, 'SCOTT', 'ANALYST',  3000,    20);
INSERT INTO EMP VALUES (9, 'KING',  'PRESIDENT',3000,    10);
INSERT INTO EMP VALUES (10,'TURNER','SALESMAN', 1500,    30);
INSERT INTO EMP VALUES (11,'ADAMS', 'CLERK',    1500,    20);

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

ENAME             SAL    TOPRANK
---------- ---------- ----------
WARD             1250          1
MARTIN           1250          2
TURNER           1500          3
ADAMS            1500          4
BLAKE            2850          5
CLARK            2850          6
JONES            2975          7
SCOTT            3000          8
KING             3000          9
SMITH                         10
ALLEN                         11

11 rows selected.

SQL>

SQL> select ename, sal, row_number() over (order by sal desc) toprank
  2  from emp
  3  order by sal desc;

ENAME             SAL    TOPRANK
---------- ---------- ----------
SMITH                          1
ALLEN                          2
KING             3000          3
SCOTT            3000          4
JONES            2975          5
BLAKE            2850          6
CLARK            2850          7
TURNER           1500          8
ADAMS            1500          9
MARTIN           1250         10
WARD             1250         11

11 rows selected.

SQL>