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From: Franco Bruno Borghesi <franco@akyasociados.com.ar>
To: Stéphane RIFF <stephane.riff@cerene.fr>
Cc: pgsql-sql@postgresql.org
Subject: Re: Query question
Date: Wed, 20 Apr 2005 14:35:56 -0300
Message-ID: <426692FC.6020807@akyasociados.com.ar> (raw)
In-Reply-To: <42667F3C.6080303@cerene.fr>
References: <42667F3C.6080303@cerene.fr>

If you have a row every 15 seconds, the answer is quite easy:

SELECT
	A1.date
FROM
	activity A1
	LEFT JOIN activity A2 ON (A2.date=A1.date-'15 secs'::interval)
WHERE
	A1.state<>A2.state OR A2.state IS NULL
ORDER BY 1



Now if you don't have a row every 15 seconds, the answer is a bit more 
complex (at least I couldn't think of an easier solution):

SELECT
	min(TMP2.new_date)
FROM
	(
		SELECT
			DISTINCT
			TMP.new_date,
			max(TMP.old_date) AS max_old_date
		FROM
			(
				SELECT	
					A1.id AS new_id, A1.date AS new_date, A1.state AS new_state,
					A2.id AS old_id, A2.date AS old_date, A2.state AS old_state
				FROM
					activity A1
					LEFT JOIN activity A2 ON (A2.date<A1.date)
				ORDER BY
					A1.date, A2.date DESC	
			) AS TMP
		WHERE
			TMP.old_state<>TMP.new_state OR TMP.old_state IS NULL
		GROUP BY
			TMP.new_date
	) TMP2
GROUP BY
	TMP2.max_old_date
ORDER BY 1


I've tested both queries on postgreSQL 8 with the data you provided, and 
they both work. Anyway try them with larger datasets before using them 
in real life ;-)

Hope it helps.


Stéphane RIFF wrote:

> Hi ,
>
> I have table that represent a switch activity like this :
>
> |         date                    | state  |
> | 2005-04-20 17:00:00 |   0     |
> | 2005-04-20 17:00:15 |   0     |
> | 2005-04-20 17:00:30 |   1     |
> | 2005-04-20 17:00:45 |   1     |
> | 2005-04-20 17:01:00 |   1     |
> | 2005-04-20 17:01:15 |   0     |
> | 2005-04-20 17:01:30 |   0     |
> | 2005-04-20 17:01:45 |   0     |
>
> I want to get the date of each states change but i not a sql expert.
> Can someone advices me
>
> Thanks
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings
>

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