Обсуждение: limit results to one row per foreign object

hi all, I have the following schema:

CREATE TABLE auctions ( id serial NOT NULL, user_id int4, title
varchar(255), body varchar(255), CONSTRAINT auctions_pkey PRIMARY KEY (id) )
WITHOUT OIDS;

CREATE TABLE bids ( id serial NOT NULL, auction_id int4, user_id int4,
amount float8, created_at timestamp, CONSTRAINT bids_pkey PRIMARY KEY (id) )
WITHOUT OIDS;

CREATE TABLE users ( id serial NOT NULL, username varchar(255), CONSTRAINT
users_pkey PRIMARY KEY (id) ) WITHOUT OIDS;

I'd like to return all the bids for a given auction, but limit it to only
the *latest* bid from each user. so regardless of how many bids a user has
placed, only their latest is returned.

I dont have a clue where to even start with this and would appreciate some
pointers
thanks

alan

Alan Bullock wrote:
> hi all, I have the following schema:
>
> CREATE TABLE auctions ( id serial NOT NULL, user_id int4, title
> varchar(255), body varchar(255), CONSTRAINT auctions_pkey PRIMARY KEY (id) )
> WITHOUT OIDS;
>
> CREATE TABLE bids ( id serial NOT NULL, auction_id int4, user_id int4,
> amount float8, created_at timestamp, CONSTRAINT bids_pkey PRIMARY KEY (id) )
> WITHOUT OIDS;
>
> CREATE TABLE users ( id serial NOT NULL, username varchar(255), CONSTRAINT
> users_pkey PRIMARY KEY (id) ) WITHOUT OIDS;
>
> I'd like to return all the bids for a given auction, but limit it to only
> the *latest* bid from each user. so regardless of how many bids a user has
> placed, only their latest is returned.
>
> I dont have a clue where to even start with this and would appreciate some
> pointers
> thanks

Hmm... No foreign key constraints?

I think you're looking for something like:

SELECT *
   FROM bids
  WHERE auction_id = 1234
  GROUP BY user_id
  HAVING created_at = MAX(created_at);

You could also use a subselect with an order by created_at DESC limit 1
over each users bids.

Regards,

--
Alban Hertroys
alban@magproductions.nl

magproductions b.v.

T: ++31(0)534346874
F: ++31(0)534346876
M:
I: www.magproductions.nl
A: Postbus 416
    7500 AK Enschede

// Integrate Your World //

Without trying it out, how about something like:

select username, maxbid
from users u, (
   select user_id, max(amount) as maxbid
   from bids
   group by user_id
   where auction_id = XXX
) as b
where u.id = b.user_id;

John


Alan Bullock wrote:
> hi all, I have the following schema:
>
> CREATE TABLE auctions ( id serial NOT NULL, user_id int4, title
> varchar(255), body varchar(255), CONSTRAINT auctions_pkey PRIMARY KEY (id) )
> WITHOUT OIDS;
>
> CREATE TABLE bids ( id serial NOT NULL, auction_id int4, user_id int4,
> amount float8, created_at timestamp, CONSTRAINT bids_pkey PRIMARY KEY (id) )
> WITHOUT OIDS;
>
> CREATE TABLE users ( id serial NOT NULL, username varchar(255), CONSTRAINT
> users_pkey PRIMARY KEY (id) ) WITHOUT OIDS;
>
> I'd like to return all the bids for a given auction, but limit it to only
> the *latest* bid from each user. so regardless of how many bids a user has
> placed, only their latest is returned.
>
> I dont have a clue where to even start with this and would appreciate some
> pointers
> thanks
>
> alan
>
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 1: if posting/reading through Usenet, please send an appropriate
>        subscribe-nomail command to majordomo@postgresql.org so that your
>        message can get through to the mailing list cleanly

If you don't mind using a (quite useful) postgres extension, this might
work for you:

select distinct on (auctions.id, users.id) *
from auctions, bids, users
where <join clauses>
order by auctions.id, users.id, created_at desc;

Hope this helps,

On Fri, Jun 30, 2006 at 04:13:12PM +0100, Alan Bullock wrote:
> hi all, I have the following schema:
>
> CREATE TABLE auctions ( id serial NOT NULL, user_id int4, title
> varchar(255), body varchar(255), CONSTRAINT auctions_pkey PRIMARY KEY (id) )
> WITHOUT OIDS;
>
> CREATE TABLE bids ( id serial NOT NULL, auction_id int4, user_id int4,
> amount float8, created_at timestamp, CONSTRAINT bids_pkey PRIMARY KEY (id) )
> WITHOUT OIDS;
>
> CREATE TABLE users ( id serial NOT NULL, username varchar(255), CONSTRAINT
> users_pkey PRIMARY KEY (id) ) WITHOUT OIDS;
>
> I'd like to return all the bids for a given auction, but limit it to only
> the *latest* bid from each user. so regardless of how many bids a user has
> placed, only their latest is returned.
>
> I dont have a clue where to even start with this and would appreciate some
> pointers
> thanks
>
> alan
>
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 1: if posting/reading through Usenet, please send an appropriate
>        subscribe-nomail command to majordomo@postgresql.org so that your
>        message can get through to the mailing list cleanly

--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.

"Alan Bullock" <liststuff@gmail.com> wrote in message
news:e83f29$dss$1@sea.gmane.org...
> hi all, I have the following schema:
>
thanks all for such prompt replies! I'm going with Martijn's solution, I'm a
total newbie and it appears simplest (though I realise it's pgsql only)