Selecting the second row of a table using rownum

Question

I have tried the below query:

select empno from (
                   select empno 
                     from emp
                    order by sal desc
                  )
where rownum = 2

This is not returning any records.

When I tried this query

 select rownum,empno from (
                        select empno from emp order by sal desc) 

It gives me this output:

ROWNUM  EMPNO      
1       7802        
2       7809    
3       7813    
4       7823

Can anyone tell me what's the problem with my first query? Why is it not returning any records when I add the ROWNUM filter?

Answer 1

To explain this behaviour, we need to understand how Oracle processes ROWNUM. When assigning ROWNUM to a row, Oracle starts at 1 and 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