SELECT salary FROM employee where rownum < 6 ORDER BY 
salary DESC
group by dept;

SELECT employee_id, employee_name, salary FROM employees
where rownum < 6 ORDER BY 
salary DESC
group by department_id;

select sal from (SELECT distinct sal FROM emp order by sal 
desc)
where rownum <6

select a.sal,a.emp_no from employee a
where 5=(select distinct(count(*) from 
employee b where a.sal> b.sal)

select * from emp e where 5> (select count(max(sal)) from emp 
    where e.deptno=deptno and sal>e.sal group by sal);

select distinct sal from emp a where 5 >(select count
(distinct sal) from emp where sal>a.sal) order by a.sal desc

Try This,

select empno,deptno,sal,dense_rank from (SELECT empno, 
deptno,sal,
DENSE_RANK() OVER (PARTITION BY
deptno ORDER BY sal DESC NULLS
LAST) DENSE_RANK
FROM emp) tmp
where dense_rank<=5;

Regards,
Murali