Re: retrieving primary key for row with MIN function

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

------------------------
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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