Clarification on “rownum”

Question

I have a table Table1

Name     Date

A        01-jun-2010

B        03-dec-2010

C        12-may-2010

When i query this table with the

Answer 1

Adding rownums is one of the last things done after the result set has been fetched from the database. This means that the first row will always have rownum 1. Rownum is better used when you want to limit the result set, for instance when doing paging.

See this for more: http://www.orafaq.com/wiki/ROWNUM

Answer 2

(Not an Oracle expert by any means)

From what I understand, rownum numbers the rows in a result set.

So, in your example:

select * from table1 where rownum=2

How many rows are there going to be in the result set? Therefore, what rownum would be assigned to such a row? Can you see now why no result is actually returned?

In general, you should avoid relying on rownum, or any features that imply an order to results. Try to think about working with the entire set of results.

With that being said, I believe the following would work:

select * from (select rownum as rn,table1.* from table1) as t where t.rn = 2

Because in that case, you're numbering the rows within the subquery.

Answer 3

Tom has an answer for many Oracle related questions

In short, rownum is available after the where clause has been applied and before the order by clause is applied.

In the case of RowNum=2, the predicate in the where clause will never evaluate to true as RowNum starts at 1 and only increases if records matching the predicate can be found.