Re: Re: "Oracle's ROWNUM"

 > If I do
 > SELECT rownum, * FROM foo WHERE rownum > 10 and rownum < 20;
 > will the output rows be numbered 1 to 9, or 11 to 19?

Such a select will never return any rows.  Essentially rownum in a where
predicate can only be used in the forms: rownum = 1; rownum < n; rownum
<= n.
Anything else will return no rows.  For example rownum = 2 will return
no rows because the first row returned by the query has by definition a
rownum of 1, but the where predicate prevents this row from being
returned, thus it can never get to a rownum value of 2 to satisfy the
where predicate.

In Oracle 8.1 they began allowing order by in the from clause to support
the top-n type selects.  Thus begining in 8.1 a query of the form
"select  * from (select foo from bar order by foo) where rownum < 10"
  became possible.  Before 8.1 it wasn't legal to have an order by in
this position, meaning you couldn't get a top-n result that was ordered.

thanks,
--Barry

Tom Lane wrote:

> Hiroshi Inoue <Inoue@tpf.co.jp> writes:
>
>>Oracle doc says.
>>
>
>>If you embed the ORDER BY clause in a subquery and place the ROWNUM
>>condition in the top-level query, you can force the ROWNUM condition
>>to be applied after the ordering of the rows. For example, the
>>following query returns the 10 smallest employee numbers. This
>>is sometimes referred to as a "top-N query":
>>
>
>>SELECT * FROM
>>   (SELECT empno FROM emp ORDER BY empno)
>>   WHERE ROWNUM < 11;
>>
>
> This thing gets more poorly-defined every time I hear about it!?
>
> Based on what's been said so far, ROWNUM in a WHERE clause means
> something completely different from ROWNUM in the SELECT target list:
> it seems they mean input row count vs output row count, respectively.
> If I do
>     SELECT rownum, * FROM foo WHERE rownum > 10 and rownum < 20;
> will the output rows be numbered 1 to 9, or 11 to 19?  If I add a
> condition, say "AND field1 < 100", to the WHERE clause, does the rownum
> count include the rows rejected by the additional clause, or not?
> And how do you justify any of these behaviors in a coherent fashion?
>
> Dare I ask how it behaves in the presence of GROUP BY, HAVING,
> aggregates, DISTINCT, UNION, ... ?
>
>             regards, tom lane
>
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