I suppose null is 'absence of value' rather than any
value (like an empty string) that can be compared with any
other value for uniqueness.
Mike.
> -----Original Message-----
> From: pgsql-admin-owner@postgresql.org
> [mailto:pgsql-admin-owner@postgresql.org]On Behalf Of P G
> Sent: Tuesday, April 08, 2003 2:14 PM
> To: pgsql-admin@postgresql.org
> Subject: [ADMIN] How does PostgreSQL treat null values in unique
> composite constraints???
>
>
> I have the following table with a unique constraint.
>
> create table testt (
> id int,
> aa int,
> bb int,
> constraint pk_testt primary key (id),
> constraint cons_testt unique (aa,bb));
>
> Let's say I insert the following:
>
> insert into testt values (1,2,null);
> insert into testt values (2,2,null);
>
> I thought this should fail because of the unique
> constraint, but it doesn't. How does PostgreSQL view
> the null values in this constraint? Will someone
> explain why I am allowed to make this insertion?
>
> TIA.
>
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