select * from emp where rownum <= 1;

select *
from emp e,
(select x.emp_number,rownum rnum
 from
 (select emp_number from emp
  order by emp_number) x) y
where y.emp_number = e.emp_number
and y.rnum = 5;

select *
from emp e,
(select x.emp_number,x.row_id,rownum rnum
 from
 (select emp_number,rowid row_id from emp
  order by emp_number) x) y
where y.row_id = e.rowid
and y.rnum = 5;

SQL> select *
  2  from emp e,
  3  (select x.empno,x.row_id,rownum rnum
  4   from
  5   (select empno,rowid row_id from emp
  6    order by empno) x) y
  7  where y.row_id = e.rowid
  8  and y.rnum = 5
  9  /
     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO      EMPNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- ----------
ROW_ID                   RNUM
------------------ ----------
      7654 MARTIN     SALESMAN        7698 28/09/1981 00:00:00       1250       1400         30       7654
AAAMTdAAEAAAAAkAAE          5

1 row selected.

select *
from emp e,
(select x.empno,x.rowid,rownum rnum
 from
 (select empno,rowid row_id from emp
  order by empno) x) y
where y.rowid = e.rowid
and y.rnum = 5;

SELECT *
FROM   emp e
     , (SELECT x.empno
             , x.row_id
             , ROWNUM rnum
        FROM   (SELECT   empno
                       , ROWID row_id
                FROM     emp
                ORDER BY empno) x) y
WHERE  y.row_id = e.ROWID AND y.rnum = 5
/

Plan
SELECT STATEMENT  CHOOSE
Cost: 5  Bytes: 1,106  Cardinality: 14  					
	6 HASH JOIN  Cost: 5  Bytes: 1,106  Cardinality: 14  				
		1 TABLE ACCESS FULL SCOTT.EMP Cost: 2  Bytes: 644  Cardinality: 14  			
		5 VIEW SCOTT. Cost: 2  Bytes: 462  Cardinality: 14  			
			4 COUNT  		
				3 VIEW SCOTT. Cost: 2  Bytes: 280  Cardinality: 14  	
					2 INDEX FULL SCAN UNIQUE SCOTT.EMP_PRIMARY_KEY Cost: 1  Bytes: 140  Cardinality: 14  

SELECT *
FROM   (SELECT empno
             , ename
             , job
             , mgr
             , hiredate
             , sal
             , comm
             , deptno
             , RANK () OVER (ORDER BY empno) rnum
        FROM   emp) x
WHERE  x.rnum = 5;

Plan
SELECT STATEMENT  CHOOSE
Cost: 2  Bytes: 1,54  Cardinality: 14  				
	4 VIEW SCOTT. Cost: 2  Bytes: 1,54  Cardinality: 14  			
		3 WINDOW NOSORT  Cost: 2  Bytes: 546  Cardinality: 14  		
			2 TABLE ACCESS BY INDEX ROWID SCOTT.EMP Cost: 2  Bytes: 546  Cardinality: 14  	
				1 INDEX FULL SCAN UNIQUE SCOTT.EMP_PRIMARY_KEY Cost: 1  Cardinality: 14