From: Andreas Joseph Krogh <andreak@officenet.no>
Organization: OfficeNet AS
To: pgsql-sql@postgresql.org
Subject: Re: Returning array of IDs as a sub-query with group-by
Date: Sat, 25 Aug 2007 17:55:30 +0200
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On Saturday 25 August 2007 17:10:57 Andreas Joseph Krogh wrote:
> Hi all.
> I have the following schema:
>
> create table item(
> id serial primary key
> );
>
>
> create table item_log(
> id serial primary key,
> item_id integer not null references item(id),
> price numeric NOT NULL
> );
>
>
> insert into item(id) values(1);
> insert into item(id) values(2);
> insert into item(id) values(3);
> insert into item(id) values(4);
>
> insert into item_log(item_id, price) values(1, 100);
> insert into item_log(item_id, price) values(1, 100);
> insert into item_log(item_id, price) values(1, 100);
> insert into item_log(item_id, price) values(1, 200);
> insert into item_log(item_id, price) values(1, 200);
> insert into item_log(item_id, price) values(1, 200);
> insert into item_log(item_id, price) values(1, 200);
> insert into item_log(item_id, price) values(1, 200);
> insert into item_log(item_id, price) values(1, 200);
> insert into item_log(item_id, price) values(2, 200);
> insert into item_log(item_id, price) values(2, 200);
>
> Now, to get out all log-entries grouped on price with count the following
> query gives me what I want
>
> SELECT COUNT(il.price), i.id AS item_id, il.price FROM item i, item_log il
> WHERE i.id =3D il.item_id GROUP BY il.price, i.id;
>
>  count | item_id | price
> -------+---------+-------
>      3 |       1 |   100
>      6 |       1 |   200
>      2 |       2 |   200
> (3 rows)
>
> Now - I would like to return an ARRAY of item_log.id for each of the two
> rows. The result I'm looking for would look like this:
>
>  count | item_id | price | item_id_array
> -------+---------+-------+---------------
>      3 |       1 |   100 | {1,2,3}
>      6 |       1 |   200 | {4,5,6,7,8,9}
>      2 |       2 |   200 | {10,11}
>
> I tried this query which complains about an ungruoped column:
>
> SELECT COUNT(il.price), i.id AS item_id, il.price,
>  ARRAY(SELECT a.id FROM item_log a WHERE a.id =3D il.id) AS item_id_array
>   FROM item i, item_log il WHERE i.id =3D il.item_id GROUP BY il.price, i=
=2Eid;
>
> ERROR:  subquery uses ungrouped column "il.id" from outer query
>
> Any hints?

I found the following CREATE AGGREGATE suggestion in the PG-docs:

CREATE AGGREGATE array_accum (anyelement)
(
    sfunc =3D array_append,
    stype =3D anyarray,
    initcond =3D '{}'
);

With this I can easily issue:
SELECT COUNT(il.price), i.id AS item_id, il.price,
 array_accum(il.id) AS item_id_array
  FROM item i, item_log il WHERE i.id =3D il.item_id GROUP BY il.price, i.i=
d;

Which does what I want:
 count | item_id | price | item_id_array
=2D------+---------+-------+---------------
     3 |       1 |   100 | {1,2,3}
     6 |       1 |   200 | {4,5,6,7,8,9}
     2 |       2 |   200 | {10,11}
(3 rows)

If someone knows of a way without introducing a new AGGREGATE I'm still=20
interrested.

=2D-=20
Andreas Joseph Krogh <andreak@officenet.no>
Senior Software Developer / Manager
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