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From: Josh Berkus <josh@agliodbs.com>
Organization: Aglio Database Solutions
To: Scott Gerhardt <scott@g-it.ca>
Subject: Re: Aggregate query for multiple records
Date: Fri, 27 Aug 2004 11:59:26 -0700
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Scott,

> >> I tried your query but it doesn't seem to work.  The interpreter
> >> expects prd2.date to be grouped but this doesn't make sence, see
> >> below:
> >
> > Oh, yeah, darn it.
> >
> > Hmmm ... maybe you could explain the purpose of selecting just 6?
> > This query
> > is going to get a *lot* uglier if there isn't another way to
> > accomplish it.
>
> The purpose of selecting the first 6 is we need sum values for just the
> first 3, 6 and 12 months oil/water production for every oil well in the
> database to perform statistical calculations.  Optionally, we also need
> the _last_ 6 months of oil and water production.
> The prd_data table has total oil for each month but some wells have 500
> months of data, we just want the first 6.

Well, here's my suggestion, to make this query both easier and faster:

1)  Add a column called "month_prod", integer, to the table.  This is the 
"number of months in production".
2) Populate this query with a calculated difference on your "date" column 
against the first appearance of that WID (I'm assuming that each increment of 
"date" = 1 month)

UPDATE prd_data SET months_prod = prd_data."date" - prd2."date" + 1
FROM prd_data prd2
WHERE prd_data.wid = prd2.wid
AND prd2."date" = ( SELECT "date" FROM prd_data prd3
	WHERE prd3.wid = prd2.wid
	ORDER BY "date" LIMIT 1 );

(warning: the above will take a long time and floor your server.  Make sure to 
do VACUUM FULL ANALYZE prd_data afterwards.)
(if you can't get the above to complete, which is possible depending on your 
hardware, you could do it procedurally in a programmming language)

3) Then running your aggregate becomes very easy/fast:

SELECT wid, sum(oil) as tot_oil, sum(hours) as tot_hours
FROM prd_data
WHERE months_prod < 7
GROUP BY wid
ORDER BY wid;

-- 
Josh Berkus
Aglio Database Solutions
San Francisco