Re: Retrieving value of column X days later

Thank you Sándor. Will experiment with this over the next couple of days.

On 7 August 2016 at 21:05, Sándor Daku <daku(dot)sandor(at)gmail(dot)com> wrote:
> 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
>