Comments on: Date handling and limit records found in Oracle

By: Igor

Igor — Fri, 02 May 2008 19:45:41 +0000

Does any one know how Oracle behaves with multiple records for the same date?
Ex.
select * from
(
select * from table order by DateOfBirth
)
Where rownum <11;

Where I have 5 records with the same DOB.

By: Nitai Aventaggiato

Nitai Aventaggiato — Wed, 09 Jan 2008 13:54:49 +0000

This post was missing on our migration to Wordpress. Sorry for that.

By: Mitch WRobel

Mitch WRobel — Thu, 21 Jun 2007 03:56:58 +0000

This was very helpful. This is the only place on the web to find a "limit" equivalent that doesn't use the rownum. My problem was date related and you hit it right on the nose with your example. Excellent! I give you mad props!

select

where

* from (select order_number , ordered_date from oe_order_headers_all order by least(ordered_date)) rownum < 21

By: Julian Lawton

Julian Lawton — Thu, 13 Jul 2006 17:52:42 +0000

One of the cool things about using an outer ROWNUM on an ordered select is that it changes how Oracle optimises it's sorting.

If we have a set of 70000 records and want the most recent 100, then doing

SELECT * FROM table ORDER BY my_date

and letting the client side code only pull 100 rows from the result set / cursor, still causes Oracle to sort a result set of 70,000 rows.

SELECT * FROM

(

SELECT * FROM table ORDER BY my_date

)

WHERE rownum <= 100

means that when it does the sort, it only keeps the top 100 results at any point, which can make huge improvements in performance (cuts down on the risk of having to page out huge temporary sort lists).

(It's typical of most client-side language programmers to dismiss this feature and just trust JDBC or cursors to do the job).