Place NULLS FIRST in 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 NULLS FIRST) toprank
  2  from emp
  3  order by sal;

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

11 rows selected.

SQL>