Subject: Re: join tables by nearest timestamp
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From: Achilleas Mantzios <achill@matrix.gatewaynet.com>
Message-ID: <53afa96e-2c45-bb10-51f3-2a0640c5d743@matrix.gatewaynet.com>
Date: Wed, 1 Nov 2017 10:11:36 +0200
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On 01/11/2017 10:06, Achilleas Mantzios wrote:
> 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;
oopss, sorry I forgot, you'll have to add a DISTINCT ON and order by l2.logtime in order to have what you want :

SELECT DISTINCT ON (l1.logtime,l1.category,l1.username,l1.action) 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,l1.category,l1.username,l1.action,l2.logtime;


>
>>
>> Thanks in advance,
>> Brice
>
>

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


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