Re: [SQL] Query question

Wed, 20 Apr 2005 10:39:22 -0700

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
  

  

  

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