Date: Wed, 17 Nov 2010 17:02:16 +0100
From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-sql@postgresql.org
Subject: Re: obtaining difference between minimum value and next in
	size
Message-ID: <20101117160216.GA32573@tux>
References: <3BC753A3784F488F98304300B6E437FD@squarepi.com>
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John Lister <john.lister-ps@kickstone.com> wrote:

> Hi, I was wondering if it is possible to do this with a single query rather
> than iterate over all of the rows in an application:
>  
> I have a table which for brevity looks like:
> create table offers {
>   integer id;
>   integer product_id;
>   double price;
> }
>  
> where for each product there is a number of offers in this table. Now my
> question:
> Is it possible to obtain the difference between just the minimum price and the
> next one up per product, so say I have the following data:
> id, product_id, price
> 123, 2, 10.01
> 125, 2, 10.05
> 128, 2, 11.30
> 134, 3, 9.45
> 147, 3, 11.42
> 157, 3, 12.08
> 167, 3, 12.09
>  
> then I would like the following returned
> product_id, difference
> 2, .04   (10.05-10.01)
> 3, 1.97 (11.42-9.45)
>  
> ,etc
>  
>  
> Any ideas?

Sure, as Tom Lane pointed out, with >= 8.4:

test=*# select * from offers ;
 id  | product_id | price
-----+------------+-------
 123 |          2 | 10.01
 125 |          2 | 10.05
 128 |          2 | 11.30
 134 |          3 |  9.45
 147 |          3 | 11.42
 157 |          3 | 12.08
 167 |          3 | 12.09
(7 Zeilen)

Zeit: 0,204 ms
test=*# select product_id, price, price - lag(price) over (partition by
product_id order by product_id, price), row_number() over (partition by
product_id)from offers;
 product_id | price | ?column? | row_number
------------+-------+----------+------------
          2 | 10.01 |          |          1
          2 | 10.05 |     0.04 |          2
          2 | 11.30 |     1.25 |          3
          3 |  9.45 |          |          1
          3 | 11.42 |     1.97 |          2
          3 | 12.08 |     0.66 |          3
          3 | 12.09 |     0.01 |          4
(7 Zeilen)

Zeit: 0,415 ms
test=*# select product_id, price, difference from (select product_id,
price, price - lag(price) over (partition by product_id order by
product_id, price) as difference, row_number() over (partition by
product_id) from offers) foo where row_number <= 2;
 product_id | price | difference
------------+-------+------------
          2 | 10.01 |
          2 | 10.05 |       0.04
          3 |  9.45 |
          3 | 11.42 |       1.97
(4 Zeilen)


Andreas
-- 
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"If I was god, I would recompile penguin with --enable-fly."   (unknown)
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