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Re: retrieving primary key for row with MIN function

From: Sean Davis <sdavis2(at)mail(dot)nih(dot)gov>
To: Adam Ruth <adamruth(at)mac(dot)com>
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: retrieving primary key for row with MIN function
Date: 2009-04-30 02:28:54
Message-ID: 264855a00904291928l3f826075n746d4aa9efc14ebd@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-novice
On Wed, Apr 29, 2009 at 6:20 PM, Adam Ruth <adamruth(at)mac(dot)com> wrote:

> Without windowing functions, I'm not sure of a built in method. But you
> could create your own aggregate.
>

Seems like you should be able to do this with a correlated subquery that
does an order and limit 1 and not have to use a custom aggregate?

Sean


>
> ------------------------
> create type top_id_type as (id int, date timestamp);
>
> create or replace function top_id_state ( state top_id_type, id int, date
> timestamp) returns top_id_type as $$
> declare
> result top_id_type;
> begin
> if state is null or date < state.date then
> result.id = id;
> result.date = date;
> else
> result = state;
> end if;
> return result;
> end
> $$ language plpgsql;
>
> create or replace function top_id_final (state top_id_type) returns int as
> $$
> begin
> return state.id;
> end
> $$ language plpgsql;
> create aggregate top_id (int, timestamp) (
> sfunc = top_id_state,
> stype = top_id_type,
> finalfunc = top_id_final
> );
> -----------------------
>
> This is my first ever user defined aggregate, so someone may be able to
> improve it.
>
> ------------------------- Usage
>
> SELECT h.id AS host_id, MIN(r.start_date) AS reservation_start_date,
> top_id(r.id, r.start_date) AS reservation_id
> FROM hosts h
> LEFT OUTER JOIN reservation_hosts rh ON rh.host_id = h.id
> LEFT OUTER JOIN reservation r ON r.id = rh.reservation_id AND
> (r.start_date, r.end_date) OVERLAPS ('2009-04-29'::date,
> '2010-04-29'::date)
> GROUP BY h.id
> ORDER BY reservation_start_date ASC
>
> -------------------------
>
> On 29/04/2009, at 10:37 PM, Marcin Krol wrote:
>
> Hello everyone,
>
> I need to retrieve PK (r.id in the query) for row with
> MIN(r.start_date), but with a twist: I need to select only one record,
> the one with minimum date.
>
> Doing it like this does not solve the problem:
>
> SELECT h.id AS host_id, MIN(r.start_date) AS reservation_start_date,
> r.id AS reservation_id
> FROM hosts h
> LEFT OUTER JOIN reservation_hosts rh ON rh.host_id = h.id
> LEFT OUTER JOIN reservation r ON r.id = rh.reservation_id AND
> (r.start_date, r.end_date) OVERLAPS ('2009-04-29'::date,
> '2010-04-29'::date)
> GROUP BY h.id, r.id
> ORDER BY reservation_start_date ASC
>
> I have to use either GROUP BY r.id or use MIN(r.id). MIN(r.id) doesn't
> select the id from the row with corresponding MIN(r.start_date), so it's
> useless, while GROUP BY r.id produces more than one row:
>
> host_id reservation_start_date reservation_id
> 361     2009-05-11              38
> 361     2009-05-17              21
>
> I need to select only row with reservation_id = 38.
>
> I would rather not do subquery for every 'host' record, since there can
> be a lot of them...
>
> Regards,
> mk
>
>
>
>
> --
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>
>

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