To: fmiddleton@verizon.net
Cc: pgsql-sql@postgresql.org
Subject: Re: redundancy in CHECK CONSTRAINTs 
In-reply-to: <43361AB7.2020701@verizon.net> 
References: <43361AB7.2020701@verizon.net>
Comments: In-reply-to Ferindo Middleton Jr <fmiddleton@verizon.net>
	message dated "Sat, 24 Sep 2005 23:34:15 -0400"
Date: Sat, 24 Sep 2005 23:58:21 -0400
Message-ID: <26530.1127620701@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Ferindo Middleton Jr <fmiddleton@verizon.net> writes:
> I have the following table:

> CREATE TABLE gyuktnine (
>      id               SERIAL,
>     intsystem      INTEGER NOT NULL REFERENCES yuksystems(id) CONSTRAINT 
> int_cannot_equal_ext
>                        CHECK (intsystem != extsystem),
>     extsystem     INTEGER NOT NULL REFERENCES yuksystems(id) CONSTRAINT 
> ext_cannot_equal_int
>                         CHECK (extsystem != intsystem), 
>     PRIMARY KEY (intsystem, extsystem)
> );

> Is this redundant?

Yes.  I think it's poor style too: a constraint referencing multiple
columns should be written as a table constraint not a column constraint.
That is, you ought to write

CREATE TABLE gyuktnine (
    id            SERIAL,
    intsystem     INTEGER NOT NULL REFERENCES yuksystems(id),
    extsystem     INTEGER NOT NULL REFERENCES yuksystems(id),
    PRIMARY KEY (intsystem, extsystem),
    CONSTRAINT int_cannot_equal_ext CHECK (intsystem != extsystem)
);

At least in the earlier versions of the SQL standard, it was actually
illegal for a column constraint to reference any other columns.  I'm not
sure if that's still true in the latest spec.  Postgres treats column
constraints and table constraints alike, but other SQL databases are
likely to be pickier.

BTW, is there any actual need for the "id" column here, seeing that
you have a natural primary key?

			regards, tom lane