On 7 August 2016 at 21:23, Tim Smith <randomdev4+postgres@xxxxxxxxx> 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
