From: Josh Berkus <josh@agliodbs.com>
To: Jan Wieck <JanWieck@Yahoo.com>
Cc: Markus Bertheau <twanger@bluetwanger.de>
Cc: olly@lfix.co.uk
Cc: pgsql-sql@postgresql.org
Subject: Re: multi column foreign key for implicitly unique columns
Date: Wed, 18 Aug 2004 10:05:13 -0700
Message-ID: <200408181005.13230.josh@agliodbs.com> (raw)
In-Reply-To: <41235A90.1080300@Yahoo.com>
References: <1092734724.2627.4.camel@dicaprio.akademie1.de>
<200408171945.15663.josh@agliodbs.com>
<41235A90.1080300@Yahoo.com>
Jan,
> In the case that a table constraint is a referential constraint,
> the table is referred to as the referencing table. The referenced
> columns of a referential constraint shall be the unique columns of
> some unique constraint of the referenced table.
Missed that one. Interesting. AFAIK, the uniqueness of referenced columns is
NOT a requirement of Relaitonal Algebra. So why does SQL require it?
Maybe I'll ask Joe Celko after he finishes moving to Austin.
I have my own issue that forced me to use triggers. Given:
table users (
name
login PK
status
etc. )
table status (
status
relation
label
definition
PK status, relation )
the relationship is:
users.status = status.status AND status.relation = 'users';
This is a mathematically definable constraint, but there is no way in standard
SQL to create an FK for it. This is one of the places I point to whenever
we have the "SQL is imperfectly relational" discussion.
--
Josh Berkus
Aglio Database Solutions
San Francisco
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To: pgsql-sql@postgresql.org
Cc: josh@agliodbs.com, JanWieck@Yahoo.com, twanger@bluetwanger.de, olly@lfix.co.uk
Subject: Re: multi column foreign key for implicitly unique columns
In-Reply-To: <200408181005.13230.josh@agliodbs.com>
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