Re: obtaining difference between minimum value and next in size

John Lister <john(dot)lister-ps(at)kickstone(dot)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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