Unique cosntraint based on contents of a field?

On Wed, Apr 7, 2010 at 10:30 PM, Warren Bell <warrenbell2@gmail.com> wrote:

Is there a way to create a unique constraint based on the content of a
field? For instance, say you have an integer field where you only want one
record with the number 1 in that field but there can be many records with
say the number 2 or any other single number in that field.

create unique index unique_field_idx
on table1(field)
where field = 1;

--
Atentamente,
Jaime Casanova
Soporte y capacitación de PostgreSQL
Asesoría y desarrollo de sistemas
Guayaquil - Ecuador
Cel. +59387171157

Jaime Casanova wrote:

On Wed, Apr 7, 2010 at 10:30 PM, Warren Bell <warrenbell2@gmail.com> wrote:

Is there a way to create a unique constraint based on the content of a
field? For instance, say you have an integer field where you only want one
record with the number 1 in that field but there can be many records with
say the number 2 or any other single number in that field.

create unique index unique_field_idx
on table1(field)
where field = 1;

Wow, great idea. I never thought of partial indexes as a method for
unique qualification. I am glad to see our documentation mentions this
usage:

http://www.postgresql.org/docs/8.4/static/sql-createindex.html

Another possible application is to use WHERE with UNIQUE to enforce
uniqueness over a subset of a table.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com