From: | Mike Orr <sluggoster(at)gmail(dot)com> |
---|---|
To: | pgsql-general(at)postgresql(dot)org |
Subject: | Counting records in a child table |
Date: | 2011-03-31 18:48:40 |
Message-ID: | AANLkTimUAJ8b5xKMDwzzDwYPSrhTQSx+iWbOdX87P36G@mail.gmail.com |
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Lists: | pgsql-general |
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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