help with uniq index

can you all help me with this:

---------- Forwarded message ----------
Date: Thu, 23 Jul 1998 12:50:17 -0600 (MDT)
From: Lorenzo Huerta <lorenzo@nmsu.edu>
To: pgsql-sql@postgreSQL.org
Cc: pgsql-general@postgreSQL.org
Subject: help with uniq index

is there a way to create a unique index on a field (in ver 6.3.1

)and also

have a single non-uniq value in that value, ie the NULL value?

Thanks,

__________________________________________________________________

* Lorenzo J. Huerta

*

* Programming Assistant

*

* Computing & Networking Networking Architecture and

Operations *

* New Mexico State University

*

* phone: (w)(505)646-2582 (h)(505) 521-8699 fax:(505) 646-8139

*

* e-mail: lorenzo@nmsu.edu

*

* Web Site: http://web.nmsu.edu/~ljhuerta

*

__________________________________________________________________

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Lorenzo Huerta wrote:

can you all help me with this:

is there a way to create a unique index on a field
(in ver 6.3.1) and also
have a single non-uniq value in that value, ie the NULL value?

Why do you ask? Why don't you try first?

vac=> create table t (x int);
CREATE
vac=> create unique index ti on t (x);
CREATE
vac=> insert into t values (1);
INSERT 18444 1
vac=> insert into t values (1);
ERROR: Cannot insert a duplicate key into a unique index
vac=> insert into t values (null);
INSERT 18446 1
vac=> insert into t values (null);
INSERT 18447 1
vac=> insert into t values (null);
INSERT 18448 1

Vadim