Oracle/Sql: Why Does Query "Select * from Records Where Rownum >= 5 and Rownum <= 10" - Return Zero Rows

Oracle/SQL: Why does query SELECT * FROM records WHERE rownum = 5 AND rownum = 10 - return zero rows

In Oracle, Rownum values are assigned after the filtering stage of the query - they are not rows of the table, they are rows of the query result set.

So the first row that comes back will always be given rownum 1, the second row that comes back rownum 2, etc.

The rownum value is only incremented after it is assigned, so any query like

select * from t where ROWNUM > 1

will never return any results. This query says 'I dont want to see the first row that gets returned to me, only the ones after that' which is sortof a paradox so nothing gets returned.

See Ask Tom:On ROWNUM and Limiting Results for more details.

rownum issue in oracle query

Please use the thread below. Pretty good explanations.
https://community.oracle.com/thread/210143
Here are two working solutions.

1. Use the row_number function for 8i and 9i :
   
   ROW_NUMBER() OVER (ORDER BY ASC) AS ROW_NUMBER.
2. Reuse Rownum pseudo column (This seems to be better)
   SELECT *
   FROM (
   SELECT t.*, ROWNUM AS rn
   FROM (
   SELECT *
   FROM mytable
   ORDER BY
   paginator, id
   ) t
   )
   WHERE rn BETWEEN 1 AND 3

Table rownum returns 0 records if rownum is tested for value greater than 100

rownum is only evaluated AFTER the row is fetched and other predicates are evaluated. that is the key. so if you say
select *
from table where rownum >= 2;

it can never work, as it works like

open cursor loop
  fetch row (rownum is evaluated as the last thing here)
    if fetched then rownum = rownum + 1
end cursor

if you request rownum starting from > 1 then its never true. to do pagination if you need to you have to code like

select *
  from (select rownum r, t.*
          from your query tables
         order by ..)
 where r>=101 
   and rownum <= 400

also read more here at ask tom

why we cannot use condition rownum=5 in oracle

You can do something similar like this:

select * from (
  select
    emp.*,
    row_number() over (order by fieldname) as rnum
  from emp
) tmp
where
  rnum between 5 and 10;

Selecting the second row of a table using rownum

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

How to use Oracle ORDER BY and ROWNUM correctly?

The where statement gets executed before the order by. So, your desired query is saying "take the first row and then order it by t_stamp desc". And that is not what you intend.

The subquery method is the proper method for doing this in Oracle.

If you want a version that works in both servers, you can use:

select ril.*
from (select ril.*, row_number() over (order by t_stamp desc) as seqnum
      from raceway_input_labo ril
     ) ril
where seqnum = 1

The outer * will return "1" in the last column. You would need to list the columns individually to avoid this.

Why = operator doesn't work with ROWNUM other than for value 1?

Because row numbers are assigned sequentially to the rows that are fetched and returned.

Here's how that statement of yours works. It grabs the first candidate row and temporarily gives it row number 1, which doesn't match your condition so it's thrown away.

Then you get the second candidate row and it's also given row number 1 (since the previous one was tossed away). It doesn't match either.

Then the third candidate row ... well, I'm sure you can see where this is going now. In short, you will never find a row that satisfies that condition.

Row numbers are only useful for = 1, < something or <= something.

This is all explained in the Oracle docs for the rownum pseudo-column.

You should also keep in mind that SQL is a relational algebra that returns unordered sets unless you specify an order. That means row number ten may be something now and something else in three minutes.

If you want a (kludgy, admittedly) way to get the nth row, you can use something like (for the fifth row):

select * from (
    select * from (
        select col1, col2, col3 from tbl order by col1 asc
    ) where rownum < 6 order by col1 desc
) where rownum = 1

The inner select will ensure you have a consistent order on the query before you start throwing away rows, and the middle select will throw away all but the first five rows from that, and also reverse the order.

The outer select will then only return the first row of the reversed set (which is the last row of the five-row set when it was in ascending order).

A better way is probably:

select * from (
    select rownum rn, col1, col2, col3 from tbl order by col1
) where rn = 5

This works by retrieving everything and assigning the rownum to a "real" column, then using that real column number to filter the results.

Selecting the second row of a table using rownum

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

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