David Newall wrote:
> PostgreSQL version: 7.4.3 (RPMs from ftp.au.postgresql.org)
>
> Operating Sysem: Fedora Core 1
>
> CREATE TABLE t(i integer UNIQUE);
> INSERT INTO t VALUES (null);
> INSERT INTO t VALUES (null);
> SELECT coalesce(i,-999) FROM t;
> coalesce
> ----------
> -999
> -999
> (2 rows)
As someone says NULL = NULL is false
so that two rows are different. If you want enforce
only one null value for that column you have to write
you own trigger.
Regards
Gaetano Mendola