Re: Counting records in a child table

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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