Thread: quastions about primary key

Is that possible to have a two columns primary key on a table with null
value on second column?

Jack

jack wrote:
> Is that possible to have a two columns primary key on a table with null
> value on second column?
> 
> Jack
Probably not, because (1,2,null,null) is unique for postresql.
Watch discussion on mailing list
about unique indexes (on which primary key is based) several days ago.

Regards,
Tomasz Myrta

On Fri, 24 Jan 2003, jack wrote:

> Is that possible to have a two columns primary key on a table with null
> value on second column?

No, because primary key implies not null on all columns involved
(technically I think it's that a non-deferrable primary key implies
not null on all columns involved, but we don't support deferrable ones)

I try use uique instead of primary key. And it works, it allows null values.
Is there any other difference between primary key and unique?

Jack
----- Original Message -----
From: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>
To: "jack" <datactrl@tpg.com.au>
Cc: <pgsql-sql@postgresql.org>
Sent: Saturday, January 25, 2003 1:14 AM
Subject: Re: [SQL] quastions about primary key


> On Fri, 24 Jan 2003, jack wrote:
>
> > Is that possible to have a two columns primary key on a table with null
> > value on second column?
>
> No, because primary key implies not null on all columns involved
> (technically I think it's that a non-deferrable primary key implies
> not null on all columns involved, but we don't support deferrable ones)
>
>

On Sun, Jan 26, 2003 at 12:29:12 +1000, jack <datactrl@tpg.com.au> wrote:
> 
> I try use uique instead of primary key. And it works, it allows null values.
> Is there any other difference between primary key and unique?

I believe that the primary key is the default key for foreign key references
into a table.