Re: Retrieving value of column X days later

On 7 August 2016 at 21:23, Tim Smith <randomdev4+postgres(at)gmail(dot)com> wrote:

> Hi,
>
> Let's say I've got a table :
>
> create table test (
> when date,
> foo numeric,
> bar numeric,
> alice numeric,
> bob numeric);
>
> insert into test values ('2016-01-01',1,2,3,4);
> insert into test values ('2016-01-02',5,6,7,8);
> insert into test values ('2016-01-03',9,10,11,12);
> insert into test values ('2016-01-04',13,14,15,16);
> insert into test values ('2016-01-05',17,18,19,20);
>
>
> What I would like to do is the following :
>
>
> (1) Given "select alice,bob from test where foo=1 and bar=2" I would
> like to return the values of alice, bob *and* the value of foo four
> days later (i.e. "17" in the above example).
>
>
> (2) But there may be times where there are insufficient data points,
> and so I would want to retrieve the last available value (e.g. "select
> alice,bob from test where foo=9 and bar=10", there is 4 days hence,
> therefore it would return the last available, i.e. "17" in this
> example, even though that is only 2 days hence).
>
>
> Any ideas welcome !
>
> Thanks !
>
>
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You can do something like this:

select alice,bob,(select foo from test as t where t.when>=test.when and
t.when<=test.when+('4 day'::interval) order by when desc limit 1 ) from
test where foo=1 and bar=2;

The "t.when>=test.when" part's purpose is not to select too many records.
It works without this but you get better performance if there are many
records in your table.

Regards,
Sándor