Subject: Re: join tables by nearest timestamp
To: pgsql-sql@postgresql.org
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From: Achilleas Mantzios <achill@matrix.gatewaynet.com>
Message-ID: <282b209c-eaac-2f22-ae04-6ecfdac5284b@matrix.gatewaynet.com>
Date: Wed, 1 Nov 2017 10:06:04 +0200
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On 01/11/2017 07:53, Brice André wrote:
> Dear all,
>
> I am running a postgresql 9.1 server and I have a table containing events information with, for each entry, an event type, a timestamp, and additional information.
>
> I would want to write a query that would return all events of type 'a', but each returned entry should be associated to the neraest event of type 'b' (ideally, the nearest, non taking into account 
> if it happened before or after, but if not possible, it could be the first happening just after).
>
> By searching on the web, I found a solution base on a "LEFT JOIN LATERAL", but this is not supported by postgresql 9.1 (and I cannot update my server) :
>
> SELECT *
> FROM
> (SELECT * FROM events WHERE type = 'a' ) as t1
> LEFT JOIN LATERAL
> (SELECT * FROM events WHERE type = 'b' AND timestamp >= t1.timestamp ORDER BY timestamp LIMIT  1) as t2
> ON TRUE;
>
> Any idea on how to adapt this query so that it runs on 9.1 ? Or any other idea on how to perform my query ?
smth like :

SELECT l1.*,l2.logtime,l2.category,l2.username from logging l1 LEFT OUTER JOIN  logging l2 ON ('t') where l1.category='vsl.login' AND (l2.category IS NULL OR l2.category='vsl.SpareCases') AND 
(l2.logtime IS NULL OR l2.logtime>=l1.logtime) order by l1.logtime;

>
> Thanks in advance,
> Brice


-- 
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt


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