select * from emp x where &n=(select count(distinct sal) 
from emp y where x.sal<=y.sal)

it will prompt for the value give the values as 10 u will 
get the output

select * from emp a
where (select count(distinct sal) from emp b where
a.sal<b.sal)=&n order by sal desc
/


this is the exact query........when there are duplicate
salaries are there also........

To find nth value

select salary from emp a where &n=(select count(distinct
salary) from emp b where a.empno<=b.empno);

to find max by nth value a.empno<=b.empno
to find min by nth value a.empno>=b.empno

OR

To find 10 max records 

select * from (select * from emp order by salary desc) where
rownum<=&n;