Re: obtaining difference between minimum value and next in size

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 | 10.01125 |          2 | 10.05128 |          2 | 11.30134 |          3 |
9.45147|          3 | 11.42157 |          3 | 12.08167 |          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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unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
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