Subject: Re: Creating a foreign key on the Union of two tables
From: Rod Taylor <rbt@rbt.ca>
To: Saad Saeed <saads@umich.edu>
Cc: pgsql-sql@postgresql.org
In-Reply-To: <a811aba0.0304061353.664e2263@posting.google.com>
References: <a811aba0.0304061353.664e2263@posting.google.com>
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Date: 07 Apr 2003 12:48:56 -0400

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On Sun, 2003-04-06 at 17:53, Saad Saeed wrote:
> I have 2 tables "A" and "B" in mySQL. They both have an attribute 'id'
> which is unique between the 2 tables. I want the union of these 2
> attributes to be a foreign key in a 3rd table "C". How do I specify
> this constraing in sql (mySQL) in table C? And for complex reasons I
> cannot merge the 2 tables A and B.

Can't say for MySQL (since this is a PostgreSQL list!!!!!), but creating
a foreign key like that to 2 different tables is against spec because
one cannot create a unique constraint across 2 tables.

That said, you could create a set of pl/pgsql functions that can imitate
what a foreign key does and will work in the situation you describe --
if you're using PostgreSQL.

--=20
Rod Taylor <rbt@rbt.ca>

PGP Key: http://www.rbt.ca/rbtpub.asc

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