Re: Unique Constraint with Multiple NULLS
Date: 7 Jan 2004 00:38:48 -0800
Message-ID: <6dae7e65.0401070038.4b3c7cf_at_posting.google.com>
"David Portas" <REMOVE_BEFORE_REPLYING_dportas_at_acm.org> wrote in message news:<G_2dncia7cWZCGmiRVn-uw_at_giganews.com>...
> > What is the SQL-92 standard for allowing multiple NULL values within a
> > unique constraint?
>
>
> A UNIQUE constraint treats NULLs in the same column as equal to each other.
> In other words UNIQUE(colX) will not be satisfied if colX contains more than
> one NULL value. This is consistent with the behaviour of other grouping
> operations like GROUP BY and DISTINCT.
>
>
I was'nt aware that null was allowed for unique constraints (db2 does not allow it). Given that it is a legal construct, the following is also legal then?
create table A (
c1 int not null primary key,
c2 int,
unique (c2)
)
create table B (
c1 int not null primary key,
c2 int,
foreign key(c2) references A(c2)
)
or can you only reference a unique and not nullable column in the parent table?
Kind regards
/Lennart
[...] Received on Wed Jan 07 2004 - 09:38:48 CET