Re: check constraint

Ok, thanks.
I just did it with 3 triggers. And it seems to work well. I wanted to
avoid this, but it seems to be the best solution.


Le jeu 05/06/2003 à 23:22, Jan Wieck a écrit :
> erwan ancel wrote:
> > well, no... these are not direct foreign keys. The constraint here is
> > that for a given record of D, B pointed by A pointed by the given D must
> > be the same as B pointed by C pointed by the given D.
> >
> > This is not a foreign key, or foreign keys are much more than what I
> > thought.
>
> Looks like a foreign key around the corner to me. Regular referential
> integrity does not allow you to define that, unless you include the B
> keys referenced in A and C into separate fields in D and build multi
> column foreign keys (probably with ON UPDATE CASCADE).
>
> The problem arising from that is that you need to know the values ahead
> or do a lookup and put them into D in a BEFORE trigger.
>
> Also I see problems arising if you later want to update A and/or C. I
> haven't thought it through completely, maybe defining the constraints
> deferred can help you out of that.
>
>
> Jan
>
>
>
> >         Erwan
> >
> > Le lun 02/06/2003 à 15:08, Bruno Wolff III a écrit :
> >> On Mon, Jun 02, 2003 at 10:52:00 +0200,
> >>   erwan ancel <erwan.ancel@free.fr> wrote:
> >> > Hi,
> >> > I would like to know if it is possible to set "complex" constraints on
> >> > databases such as:
> >> >
> >> > A->B means that in table A, each record references a record of table B
> >> > (or NULL)
> >> >
> >> > so we have:
> >> >
> >> > A->B
> >> > C->B
> >> > D->C
> >> > D->A
> >> > constraint: for one record of D, D->A->B = D->C->B
> >> >
> >> >     Hope it is clear enough.
> >>
> >> It looks like you are talking about foreign keys. Postgres has foreign key
> >> constraints. You can look at the create table documentation to see how
> >> to define them when creating a table.
> >>
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>