Thread: Primary and Foreign Key?
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. Thanks! -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.11.3/107 - Release Date: 9/20/2005
In relational databases A table can have at most one primary key. However, you are not required to have a primary key, so I would just have the two foreign keys in the table PEOPLEGROUPS. For more information on Constraints see http://www.postgresql.org/docs/8.0/interactive/ddl-constraints.html Wayne Unruh From : A. Banks <abanks@insightbb.com> Sent : Wednesday, September 21, 2005 8:46 PM To : <pgsql-novice@postgresql.org> Subject : [NOVICE] Primary and Foreign Key? 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. Thanks!
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@lfix.co.uk
Isle of Wight http://www.lfix.co.uk/oliver
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========================================
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I think i found what i was looking for in the docs: 5.3.4. Primary Keys Technically, a primary key constraint is simply a combination of a unique constraint and a not-null constraint. So i suppose i suppose i would use PRIKARY KEY(peopleid,groupid) to enforce the UNIQUE NOT NULL constraint in one expression. Thanks! -----Original Message----- From: Wayne Unruh [mailto:waynepunruh@hotmail.com] Sent: Saturday, September 24, 2005 1:46 AM To: abanks@insightbb.com; pgsql-novice@postgresql.org Subject: RE: [NOVICE] Primary and Foreign Key? In relational databases A table can have at most one primary key. However, you are not required to have a primary key, so I would just have the two foreign keys in the table PEOPLEGROUPS. For more information on Constraints see http://www.postgresql.org/docs/8.0/interactive/ddl-constraints.html Wayne Unruh From : A. Banks <abanks@insightbb.com> Sent : Wednesday, September 21, 2005 8:46 PM To : <pgsql-novice@postgresql.org> Subject : [NOVICE] Primary and Foreign Key? 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. Thanks! -- No virus found in this incoming message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.11.5/110 - Release Date: 9/22/2005 -- No virus found in this incoming message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.11.5/110 - Release Date: 9/22/2005