Thanks. How would I do it with a window function? I thought windows
only compared groups of records in the same table.
On Thu, Mar 31, 2011 at 12:01 PM, David Johnston <polobo@yahoo.com> wrote:
> An alternative:
>
> SELECT
> parent.*,
> COALESCE(child.childcount, 0) AS whatever
> FROM parent
> LEFT JOIN
> (SELECT parentid, count(*) as childcount FROM child GROUP BY parented) child
> ON (parent.id = child.parentid)
>
> You could also do:
> SELECT parent.*,
> COALESCE((SELECT count(*) FROM child WHERE child.id = parent.id),0) AS
> childcount --coalesce may not be necessary....
> FROM parent
>
> Window Functions can also give appropriate results.
>
> I am not positive whether COUNT(*) excludes NULL during its count but a
> quick documentation search or just trying it will tell you that.
>
> David J.
>
> -----Original Message-----
> From: pgsql-general-owner@postgresql.org
> [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Mike Orr
> Sent: Thursday, March 31, 2011 2:49 PM
> To: pgsql-general@postgresql.org
> Subject: [GENERAL] Counting records in a child table
>
> I know how to do count(*)/group by on a single table, but how do I get a
> count of related records in a child table? Some of the counts will be zero.
>
> SELECT
> parent.id AS id,
> parent.name AS name,
> parent.create_date AS create_date,
> COUNT(child.id) AS count
> FROM parent LEFT JOIN child ON parent.id = child.parent_id GROUP BY
> parent.id, parent.name, parent.create_date ORDER by count desc;
>
> Is this correct, and is it the simplest way to do it?
>
> I used a left join to avoid skipping parent records that have no child
> records. I grouped by parent.id because those are the result rows I want. I
> added the other group by fields because psql refused to run the query
> otherwise.
>
> --
> Mike Orr <sluggoster@gmail.com>
>
> --
> Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make
> changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general
>
>
--
Mike Orr <sluggoster@gmail.com>
