Re: Counting records in a child table

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(at)yahoo(dot)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(at)postgresql(dot)org
> [mailto:pgsql-general-owner(at)postgresql(dot)org] On Behalf Of Mike Orr
> Sent: Thursday, March 31, 2011 2:49 PM
> To: pgsql-general(at)postgresql(dot)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(at)gmail(dot)com>
>
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>

--
Mike Orr <sluggoster(at)gmail(dot)com>