Retrieving value of column X days later

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

2016-08-07 22:23 GMT+03:00 Tim Smith <randomdev4+postgres@gmail.com>:

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);

I had to rename column "when" into "when_d", as I do not like quoting
identifiers.

Try this query with window functions:

SELECT *,lead(foo,4) OVER (ORDER BY when_d),
last_value(foo) OVER (ORDER BY when_d RANGE BETWEEN UNBOUNDED
PRECEDING AND UNBOUNDED FOLLOWING)
FROM test;

This will give you the ability to lookup needed values.
You'll have to use subquery though, as window functions are evaluated after
the `WHERE` clause.

--
Victor Y. Yegorov

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

On 7 August 2016 at 21:05, Sándor Daku <daku.sandor@gmail.com> wrote:

On 7 August 2016 at 21:23, Tim Smith <randomdev4+postgres@gmail.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 !

--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
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http://www.postgresql.org/mailpref/pgsql-general

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

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

On 7 August 2016 at 21:41, Victor Yegorov <vyegorov@gmail.com> wrote:

2016-08-07 22:23 GMT+03:00 Tim Smith <randomdev4+postgres@gmail.com>:

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);

I had to rename column "when" into "when_d", as I do not like quoting
identifiers.

Try this query with window functions:

SELECT *,lead(foo,4) OVER (ORDER BY when_d),
last_value(foo) OVER (ORDER BY when_d RANGE BETWEEN UNBOUNDED
PRECEDING AND UNBOUNDED FOLLOWING)
FROM test;

This will give you the ability to lookup needed values.
You'll have to use subquery though, as window functions are evaluated after
the `WHERE` clause.

--
Victor Y. Yegorov

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