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Subject: Re: Delete with join -- deleting related table entries?
Date: Wed, 8 Feb 2006 09:32:27 -0800
Message-ID: <144D12D7DD4EC04F99241498BB4EEDCC20CC56@nelson.osl.com>
Thread-Topic: [SQL] Delete with join -- deleting related table entries?
Thread-Index: AcYs0yTjwft69upxR4aOmQji7ewuHQAAcGLQ
From: "Owen Jacobson" <ojacobson@osl.com>
To: "Bryce Nesbitt" <bryce1@obviously.com>,
	<pgsql-sql@postgresql.org>

Bryce Nesbitt wrote:

> When I delete a record from a certain table, I need to delete a
> (possibly) attached note as well.  How can I do this with=20
> postgres?  The
> tables are like this:
>=20
> reservation
>     reservation_id
>     stuff...
>=20
> isuse
>     issue_id
>     reservation_id
        references reservation (reservation_id) -- ADD
>     stuff..
>=20
> note
>     issue_id
        references isuse (issue_id) -- ADD (kept typo in example)
>     text comments...
>=20
> A select that pulls out what I want to delete is:
>=20
>     SELECT reservation_id,issue_id,note_id,eg_note.comments FROM
> eg_reservation
>        LEFT JOIN eg_issue USING (reservation_id)
>        LEFT JOIN eg_note USING (issue_id)
>        WHERE reservation_id > condition;
>=20
> Can anyone help me turn this into a DELETE statement?

1.  Add foreign key references between the tables to ensure that there =
are only notes and issues (isuses? :) for existing issues and =
reservations respectively.  You can make those references 'ON DELETE =
CASCADE' so that a delete of the original reservation cascades down to =
related entries in the issue table, which in turn cascade down to the =
related entries in the note table.

2.  Or...

BEGIN;
  DELETE FROM note WHERE issue_id IN (SELECT issue_id FROM isuse
    WHERE reservation_id =3D reservation_to_delete);
  DELETE FROM isuse WHERE reservation_id =3D reservation_to_delete;
  DELETE FROM reservations WHERE reservation_id =3D =
reservation_to_delete;
END;

with an appropriate value or expression substituted into =
reservation_to_delete.  This would be the "hard way", but (as it's in a =
single transaction) will still protect other clients from seeing a =
partial delete.

Get yourself a good, non-MySQL-specific database book, which should =
explain how referential integrity is handled in databases.

-Owen