Re: Query with LIMIT clause

Firstly, who should I reply to, you or the list?
Your solution is working pretty fine, I appreciate your advice. Now, I am
sure that an index is a good idea in order to make the query fast. What
would be a good criteria to define an index? Will an index for final date
and another for initial date is the choice, or one index composed of both
initial and final date?

Regards,
Jorge Maldonado

On Sun, Sep 9, 2012 at 12:45 PM, David Johnston <polobo(at)yahoo(dot)com> wrote:

> From: pgsql-sql-owner(at)postgresql(dot)org [mailto:
> pgsql-sql-owner(at)postgresql(dot)org]
> On Behalf Of JORGE MALDONADO
> Sent: Sunday, September 09, 2012 1:26 PM
> To: pgsql-sql(at)postgresql(dot)org
> Subject: [SQL] Query with LIMIT clause
>
> I have the following records that I get from a query, the fields are date
> type in day/month/year format:
>
> -------------------------------------
> Initial Final
> Date Date
> -------------------------------------
> 27/08/2012 04/09/2012
> 29/08/2012 09/09/2012
> 28/08/2012 09/09/2012
> 30/08/2012 09/09/2012
> 30/08/2012 09/09/2012
> 27/08/2012 09/09/2012
> 31/08/2012 09/09/2012
> 28/08/2012 10/09/2012
> 05/09/2012 16/09/2012
>
> As you can see, this result is ordered by Final Date. What I need is to get
> the last 5 records only. If I use the LIMIT 5 clause I get the first 5. I
> have tried an ORDEY BY DESC but the result is the same. I will very much
> appreciate your comments.
>
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
>
> First, you should really order by both columns, not just "final date". The
> ties on 9/9/12 are unordered.
>
> Second, you will indeed need to reverse the sort order and then take the
> first 5 records; figuring out and limiting on the last 5 isn't worth the
> effort.
>
> SELECT initial_date, final_date
> FROM date_source
> ORDER BY final_date DESC, initial_date DESC
> LIMIT 5
>
> You can put the above into a sub-query and re-order if the final result is
> needed in ascending order.
>
> If this doesn't seem to work you will want to provide the exact
> query/queries you are trying so that someone may spot what you are doing
> wrong.
>
> Dave
>
>
>