From: Sebastian Ritter <ritter.sebastian@gmail.com>
To: Guy Fraser <guy@incentre.net>
Subject: Re: Was: fetch first rows of grouped data
Date: Tue, 28 Aug 2007 11:38:22 -0600
Message-ID: <99b656cb0708280931h19ca997dw68ed251beb2a99ff@mail.gmail.com> (raw)
In-Reply-To: <99b656cb0708280923p171fb6eg9a2458dc08f7e33b@mail.gmail.com>
References: <1188317103.8070.34.camel@sigurd.incentre.net>
<99b656cb0708280923p171fb6eg9a2458dc08f7e33b@mail.gmail.com>
Sorry my last post isnt complete.
This would have to be part of a sub select. It would be more lilke:
SELECT * from cities LEFT OUTER JOIN (SELECT c.id as city_id, event.id FROM
cities c LEFT OUTER JOIN events e ON (c.id = e.city_id) WHERE c.id =
cities.id ORDER BY e.date DESC LIMIT 2) as x ON (cities.id = x.city_id)
I think that would work.
Seb
On 8/28/07, Sebastian Ritter <ritter.sebastian@gmail.com> wrote:
>
> Hi There,
>
> You can do something like :
>
> SELECT * FROM cities c LEFT OUTER JOIN events e ON (c.id =e.city_id) ORDER
> BY e.date DESC LIMIT 2
>
> The left outer join here would ensure that cities with no events are also
> added to your result set.
>
> Seb
>
> On 8/28/07, Guy Fraser <guy@incentre.net> wrote:
> >
> > Hi there
> >
> > I have a list of events that take place in a certain city at a
> > certain date. Now I would like to have the first two (ordered by
> > date) events for each city.
> > Is there a way to do this with one query?
> > I am using PostgreSQL 7.4.
> >
> > Thanks for any tips.
> >
> > Claudia
> >
> >
> > I think I may have come up with a possible solution.
> >
> > Create a selection that produces a unique identifier for each city
> > ordered by date then use array_accum to collect the unique identifiers
> > for each city, then match the first two elements of the array with the
> > identifiers.
> >
> > For instance if you had a table :
> >
> > CREATE TABLE crazy_talk (
> > ct_id bigserial primary key,
> > ct_city text,
> > ct_date date,
> > ct_data text
> > ) ;
> >
> > Then you could use :
> >
> > SELECT
> > ct_id ,
> > ct_city ,
> > ct_date ,
> > ct_data
> > FROM
> > crazy_talk ,
> > (SELECT
> > ct_city AS city,
> > array_accum(ct_id) as match
> > FROM
> > crazy_talk
> > ORDER BY
> > ct_city ,
> > ct_date
> > GROUP BY
> > ct_city ) AS data_set
> > WHERE
> > ct_city = city AND
> > ct_id IN (match[0],match[1])
> > ORDER BY
> > ct_city ,
> > ct_date
> > ;
> >
> > I hope this helps, I did not try it, but I think it should work.
> >
> > PS if you don't have array_accum here it is :
> >
> > CREATE AGGREGATE array_accum (
> > BASETYPE = anyelement,
> > SFUNC = array_append,
> > STYPE = anyarray,
> > INITCOND = '{}'
> > );
> >
> >
> >
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 5: don't forget to increase your free space map settings
> >
>
>
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-sql@postgresql.org
Cc: ritter.sebastian@gmail.com, guy@incentre.net
Subject: Re: Was: fetch first rows of grouped data
In-Reply-To: <99b656cb0708280931h19ca997dw68ed251beb2a99ff@mail.gmail.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox