Re: SQL Query Newbie Help - Mailing list pgsql-sql

From Jim C. Nasby
Subject Re: SQL Query Newbie Help
Date
Msg-id 20060327134942.GK80726@pervasive.com
Whole thread Raw
In response to Re: SQL Query Newbie Help  (Stephan Szabo <sszabo@megazone.bigpanda.com>)
List pgsql-sql
On Fri, Mar 24, 2006 at 02:29:10PM -0800, Stephan Szabo wrote:
> 
> On Fri, 24 Mar 2006, Julie Robinson wrote:
> 
> > This works, but is there a better solution?
> >
> > select *
> > from quality_control_reset T
> > where date = (
> >     select max(date)
> >     from quality_control_reset
> >     where qualitycontrolrange = T.qualitycontrolrange);
> 
> If you can use PostgreSQL extensions (and don't care that you might not
> get two rows if two ids had the same date equaling the max date for a
> given range), maybe something like:
> 
> select distinct on (qualitycontrolrange) id, date, qualitycontrolrange
>  from quality_control_reset order by qualitycontrolrange,date desc;
> 
> 
> Otherwise, you might see how the above compares in plan to something like
> (not really tested):
> 
> select T.* from quality_control_reset T inner join
>  (select qualitycontrolrange, max(date) as date from quality_control_reset
>   group by qualitycontrolrange) T2
>  on (T.qualitycontrolrange = T2.qualitycontrolrange and T.date=T2.date);

BTW, I believe the new row operator fixes in 8.2 make it possible to use
them to do this kind of thing as well...
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461


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