I have tried the below query:
select empno from (
order by sal desc
where rownum = 2
select rownum,empno from (
select empno from emp order by sal desc)
To explain this behaviour, we need to understand how Oracle processes ROWNUM. When assigning ROWNUM to a row, Oracle starts at 1 and only only increments the value when a row is selected; that is, when all conditions in the WHERE clause are met. Since our condition requires that ROWNUM is greater than 2, no rows are selected and ROWNUM is never incremented beyond 1.
The bottom line is that conditions such as the following will work as expected.
.. WHERE rownum = 1;
.. WHERE rownum <= 10;
While queries with these conditions will always return zero rows.
.. WHERE rownum = 2;
.. WHERE rownum > 10;
Quoted from Understanding Oracle rownum
You should modify you query in this way in order to work:
select empno from ( select empno, rownum as rn from ( select empno from emp order by sal desc ) ) where rn=2;
EDIT: I've corrected the query to get the rownum after the order by sal desc