From: Peter Childs <Blue.Dragon@blueyonder.co.uk>
To: Adam Sherman <adam@tritus.ca>, pgsql-sql@postgresql.org
Subject: Re: Constraint Syntax Question
Date: Mon, 12 May 2003 05:47:48 +0100
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On Sunday 11 May 2003 23:17, Adam Sherman wrote:
> I have a many-to-many relationship between A & B where a B *must* relate to
> at least 1 A but an A may have zero Bs.
>
> How do add this using an ALTER TABLE statement?

	You can't postgres enforces that the foreign key must be unique. so 

ALTER TABLE B ADD CONSTRAINT bafk FOREIGN KEY (key) REFERENCES A(key);

looks like it should work by A(key) must be unique so you will get an error.
I think that this is probably bad database design and you need to get you 
database into 1st normal form hense getting read of any many to many 
relations by removing any many to many relations. Find a good book on 
database design.
	I hope that helps

Peter Childs

>
> Thanks,
>
> A.
>
> --
> Adam Sherman
> Tritus CG Inc.
> http://www.tritus.ca/
> +1 (613) 797-6819
>
>
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