From: | Thomas Kellerer <spam_eater(at)gmx(dot)net> |
---|---|
To: | pgsql-admin(at)postgresql(dot)org |
Subject: | Re: Composite Unique Key - Doubt |
Date: | 2013-06-20 08:12:47 |
Message-ID: | kpudhi$ko2$1@ger.gmane.org |
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Lists: | pgsql-admin |
Technical Doubts wrote on 20.06.2013 09:42:
> technologies
> (
> technologyid bigint,
> status character(1),
> implementeddate date
> *CONSTRAINT technologies_uq UNIQUE (technologyid, status, implementeddate)*
> )
>
> entering data as
>
> insert into technologies (technologyid,status,implementeddate)
> values
> (123,'P',null),
> (123,'P',null);
>
> 2 rows affected.
>
> table accepting duplicate values in spite of composite unique constraint..
> where I am doing wrong?
>
That's because of the null values. Any comparison with NULL yields "unknown" and in case of a
constraint this means the constraint is not violated.
Apparently you can not make that column NOT NULL (which would prevent this situation).
But you could create a unique index on an expression that treats NULL as "some value", e.g:
create table technologies
(
technologyid bigint,
status character(1),
implementeddate date
);
create unique index technologies_uq
on technologies (technologyid, status, coalesce(implementeddate, date '1900-01-01'));
A unique constraint is slightly different to a unique index (e.g. it cannot be the target
of a foreign key) but it would server your purpose in this case - unlesse you have
requirements you did not mention.
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