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retrieving primary key for row with MIN function

From: Marcin Krol <mrkafk(at)gmail(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: retrieving primary key for row with MIN function
Date: 2009-04-29 12:37:50
Message-ID: 49F84A1E.2050404@gmail.com (view raw or flat)
Thread:
Lists: pgsql-novice
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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Next:From: Sean DavisDate: 2009-04-29 13:08:21
Subject: Re: retrieving primary key for row with MIN function
Previous:From: Jasen BettsDate: 2009-04-29 12:27:39
Subject: Re: Copying data from one table of one database to other table f other database

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