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Re: Primary and Foreign Key?

From: Oliver Elphick <olly(at)lfix(dot)co(dot)uk>
To: "A(dot) Banks" <abanks(at)insightbb(dot)com>
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: Primary and Foreign Key?
Date: 2005-09-24 11:31:48
Message-ID: 1127561508.10691.11.camel@linda.lfix.co.uk (view raw or flat)
Thread:
Lists: pgsql-novice
On Wed, 2005-09-21 at 19:46 -0500, A. Banks wrote:
> I have the following related tables:
> 
> PEOPLE
> --------------
> peopleid pkey,
> name,
> etc
> 
> 
> GROUPS
> -------------
> groupid pkey,
> description,
> etc
> 
> 
> PEOPLEGROUPS
> -------------------
> peopleid pkey/fkey,
> groupid pkey/fkey
> 
> 
> What is the CORRECT way (in Postgres) to define the PEOPLEGROUPS table so
> that it has both the double primary key AND still acts as a foreign key for
> people.peopleid and groups.groupid? Can i specify both or is this not
> necessary? Any suggestions would be appreciated.

Your terminology is a bit wrong; there can only ever be one primary key
for a table; what you want is probably a single primary key made up of
two columns.

Your definition will look like this:

CREATE TABLE people (
   peopleid	SERIAL PRIMARY KEY,  -- SERIAL is an INTEGER that autoincrements
                                     -- if no value is supplied on insertion
   name		TEXT NOT NULL,
   ...
);

CREATE TABLE groups (
   groupid	SERIAL PRIMARY KEY,
   name		TEXT NOT NULL,
   ...
);

CREATE TABLE peoplegroups (
   peopleid	INTEGER
                REFERENCES people (peopleid)
                   ON UPDATE CASCADE ON DELETE NO ACTION,
   groupid	INTEGER
                REFERENCES people (peopleid)
                   ON UPDATE CASCADE ON DELETE NO ACTION,
   PRIMARY KEY (peopleid, groupid)
);
-- 
Oliver Elphick                                          olly(at)lfix(dot)co(dot)uk
Isle of Wight                              http://www.lfix.co.uk/oliver
GPG: 1024D/A54310EA  92C8 39E7 280E 3631 3F0E  1EC0 5664 7A2F A543 10EA
                 ========================================
   Do you want to know God?   http://www.lfix.co.uk/knowing_god.html


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