Re: Delete with join -- deleting related table entries?

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
> postgres?  The
> tables are like this:
>
> reservation
>     reservation_id
>     stuff...
>
> isuse
>     issue_id
>     reservation_id       references reservation (reservation_id) -- ADD
>     stuff..
>
> note
>     issue_id       references isuse (issue_id) -- ADD (kept typo in example)
>     text comments...
>
> A select that pulls out what I want to delete is:
>
>     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;
>
> 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
existingissues and reservations respectively.  You can make those references 'ON DELETE CASCADE' so that a delete of
theoriginal reservation cascades down to related entries in the issue table, which in turn cascade down to the related
entriesin the note table. 

2.  Or...

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

with an appropriate value or expression substituted into reservation_to_delete.  This would be the "hard way", but (as
it'sin 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