From: Oliveiros C, <oliveiros.cristina@marktest.pt>
To: justin <justin@emproshunts.com>
Cc: pgsql-sql@postgresql.org
Cc: gary.stainburn@ringways.co.uk
Subject: Re: simple (?) join
Date: Fri, 25 Sep 2009 12:00:17 +0100
Message-ID: <1EF7ADDE66AA4D4E8E7EB2021CAD6CBB@marktestcr.marktest.pt> (raw)
References: <200909241616.36933.gary.stainburn@ringways.co.uk>
<20090924194629.44bc236f@dwnoon.ntlworld.com>
<4ABBD34B.9080600@emproshunts.com>
Hello, Justin, Gary.
Justin, your (the second one) query is not much different from mine.
You previewed the possibility of having orders without any matching entry on orders_log with your left join, something that I haven't. Gary, will you have
records on your orders table that don't reference any record on your orders_log table? If so, Justin's query is the right one you should use.
You return the full record from orders and an additional column from orders_log, the ol_timestamp column. I understood that Gary wanted the full record from orders_log, not
just the timestamp column. That part is done by my subquery .
I think Gary could clarify what he wants exactly. Gary? :)
Also, Justin, your query design seems right to me, but maybe you should add this (the part in comment) to your subquery
SELECT MAX(ol_timestamp) /* as ol_timestamp */ , o_id FROM orders_log group by o_id
because the MAX(ol_timestamp) will receive the name max, not ol_timestamp, and probably
the parser will complain that column ol_timestamp does not exist.
Ain't I right?
Best,
Oliveiros
----- Original Message -----
From: justin
To: David W Noon
Cc: pgsql-sql@postgresql.org ; gary.stainburn@ringways.co.uk
Sent: Thursday, September 24, 2009 9:15 PM
Subject: Re: [SQL] simple (?) join
David W Noon wrote:
On Thu, 24 Sep 2009 16:16:36 +0100, Gary Stainburn wrote about [SQL]
simple (?) join:
create table orders (
o_id serial primary key
...
);
create table orders_log (
ol_id serial primary key,
o_id int4 not null references orders(o_id),
ol_timestamp timestamp,
ol_user,
);
How can I select all from orders and the last (latest) entry from the
orders_log?
SELECT * FROM orders
WHERE o_id IN (SELECT o_id FROM orders_log
WHERE ol_timestamp = (SELECT MAX(ol_timestamp) FROM orders_log));
No joins required.
I don't think that is what he is requesting. I read it he also wants the timestamp included in the result set
A nested query
Select
orders.*,
(SELECT MAX(ol_timestamp) FROM orders_log where orders_log.o_id = orders.oid)
From orders
Still another option is using a join
Select
orders.*, ol_timestamp
From orders
left join (SELECT MAX(ol_timestamp), o_id FROM orders_log group by o_id) as JoinQuery on JoinQuery.o_id = orders.o_id
The second one should be faster
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To: pgsql-sql@postgresql.org
Cc: oliveiros.cristina@marktest.pt, justin@emproshunts.com, gary.stainburn@ringways.co.uk
Subject: Re: simple (?) join
In-Reply-To: <1EF7ADDE66AA4D4E8E7EB2021CAD6CBB@marktestcr.marktest.pt>
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