Re: Primary -- foreign keys

From: volo <yariv_volovelsky_at_yahoo.com>
Date: Sat, 11 Aug 2001 01:40:15 +0200
Message-ID: <3b74700d$0$1087$9b622d9e_at_news.freenet.de>

"Pavol Obecajcik" <Pavol.Obecajcik_at_onatix.com> wrote in message news:f19254f5.0108101207.6c3e4535_at_posting.google.com...
> prk25_at_yahoo.com (keith) wrote in message
news:<adbca63f.0108100550.b176fc5_at_posting.google.com>...

...
> > create table abc
> > (
> > x integer primary key,
> > y char(20),
> > foreign key (x) references abc(x)
> > );
> >
> > What's being accomplished by creating a forein key which references a
> > primary key of the same table??? Primary and foreign keys are on the
> > same column of the same table????
> >
> > Thanks
> >
> > Keith

>
>

> Hi,
> This is nonsense :-).

>

> Primary key is (must be) UNIQUE and NOT NULL.

>


>   1.  Foreign key must hold only existing values from referenced column

>       (eventually NULL) => violated unique restriction.

> 2. NOT NULL restriction => impossible to insert first row in table

>

> try this:
> create table abc
> (
> x integer not null primary key,
> parent_abc integer,
> y char(20)
> )
> ;
> alter table abc add foreign key (parent_abc) references abc(x)
> ;

>
> Palo

Hi,

I also thought it was nonsense :-) and came theoretically to the same conclusion that you can't insert the first row. Then I tried it with personal Oracle and I was ABLE to insert some rows. So I suppose Oracle is right and we're are wrong. :-(

can someone explain the meaning of this?

Yariv Received on Sat Aug 11 2001 - 01:40:15 CEST