Thanks Dave,
I came up with the same answer after much googling. I managed to write a
trigger to do this and it worked first time :D
Then I realised I was going to have to write more triggers to handle the
reverse side, i.e. deleting a coach_train record after a booking has been
made.
Thanks anyway.
Gary
On Thursday 19 May 2011 01:09:07 David W Noon wrote:
> On Wed, 18 May 2011 20:10:19 +0100, Gary Stainburn wrote about [SQL]
>
> foreign keys and lots of tables:
> >I have the following tables (individual seat allocation removed to
> >make it simpler)
>
> Omitting details makes the problem more difficult to comprehend.
>
> [snip]
>
> >create table booking_seats (
> > b_id int4 not null references bookings(b_id),
> > c_id int4, -- carriage ID
> > c_seat varchar(10) -- seat label
> >);
> >
> >The following ensures the seat exists on the coach. (not shown)
> >
> >alter table booking_seats add constraint seat_exists
> > foreign key (c_id, c_seat) references coach_seats (c_id,c_seat);
> >
> >How would I ensure that the coach exists on the train. I would need to
> >convert the b_id to a t_id using the bookings table and I don't know
> >how.
>
> I think you will need to write a trigger procedure with something like
> the following query inside it:
>
> IF NOT EXISTS(SELECT * FROM train_coaches AS tc
> INNER JOIN bookings AS b ON b.t_id = tc.t_id
> WHERE b.b_id = NEW.b_id AND tc.c_id = NEW.c_id)) THEN
> -- Something is wrong.
>
> You cannot use a subquery in a CHECK constraint, so I think a trigger
> will be the go.
>
> >To complicate things, when the initial booking is made, bot c_id and
> >c_seat are NULL. Will this make any difference?
>
> If c_id is NULL you cannot check anything against it, so your data
> integrity has just gone for a Burton.
--
Gary Stainburn
Group I.T. Manager
Ringways Garages
http://www.ringways.co.uk
