Message-Id: <5E69BD4A-C7CC-4821-A51C-4590449F7E10@comcast.net>
From: Daryl Richter <ngzax@comcast.net>
To: PgSQL-sql <pgsql-sql@postgresql.org>
Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
Content-Transfer-Encoding: 7bit
Mime-Version: 1.0 (Apple Message framework v935.3)
Subject: Re: Foreign keys and fixed values...
Date: Thu, 30 Jul 2009 11:33:59 -0400
References: <AF3897D6-C5AB-409D-BB32-1F4FC70E2963@comcast.net>

Hi-

On Jul 29, 2009, at 4:16 PM, Kjell Rune Skaaraas wrote:

>
> Hello,
>
> I'm having a problem using foreign keys, or at least in a way i find  
> good. Basicly I have a table I wish to use as a foreign key with  
> lookup "id" and "code", like:
>
> 1,a
> 1,b
> 1,c
> 2,a
> 2,d
>
> I have of course an UNIQUE on (id,code). However, in the table I  
> want the key to apply to, I only have one column that I want to  
> restrict to one lookup. Basicly I want to do something like:
>
> ALTER TABLE data ADD FOREIGN KEY ( 2, col ) REFERENCES lookups ( id,  
> code )
>
> In this case 2 is a literal integer, not a column reference and the  
> result should be that a and d is legal - but this command isn't legal.
>
> Options:
> 1) Add another column to my data table like:
> ALTER TABLE data ADD COLUMN dummy INTEGER DEFAULT 2
> ALTER TABLE data ADD FOREIGN KEY ( dummy, col ) REFERENCES lookups  
> ( id, code )
> This works, but creates a useless column in my data table that feels  
> very unclean.
>
> 2) Refactor the lookup table to have one lookup per table. However,  
> this also means I'll probably need to duplicate translation tables,  
> value mapping tables etc. and a bunch of queries would have to  
> dynamically alter table names. It seems more than a little messy.
>


#2 is the proper solution.

A foreign key is a binding constraint between all the rows in the 2  
tables.  You are essentially trying to create a FK that applies to  
some rows and not to others.  Or, alternately, a table which has  
conditional FKs to other tables based upon the data it contains.

So, you have created a dependency between the data in the database and  
your schema which is a bad idea.  Imagine, for example, after you have  
set this all up, the user comes to you and says, "Sorry, when I told  
you 2 maps to either a or d, I should have said 3".  Now, instead of  
just issuing an update, you have to change your schema.

Your other concerns regarding this option can almost certainly be  
addressed through proper schema design too.

--
Daryl
http://itsallsemantics.com

"Everyone thinks of changing the world, but no one thinks of changing  
himself."
- Leo Tolstoy