Subject: Re: unique constraint definition within create table
To: Andreas Kretschmer <akretschmer@spamfence.net>, pgsql-sql@postgresql.org
References: <20151202063621.GA5526@tux>
From: Vik Fearing <vik@2ndquadrant.fr>
Message-ID: <566675D5.7060401@2ndquadrant.fr>
Date: Tue, 8 Dec 2015 07:16:53 +0100
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On 12/02/2015 07:36 AM, Andreas Kretschmer wrote:
> Hi @ll,
> 
> i'm trying to create a table with 2 int-columns and a constraint that a
> pair of (x,y) cannot be as (y,x) inserted:
> 
> test=# create table foo(u1 int,u2 int, unique (least(u1,u2),greatest(u1,u2)));
> ERROR:  syntax error at or near "("
> LINE 1: create table foo(u1 int,u2 int, unique (least(u1,u2),greates...
> 
> 
> I know, i can solve that in this way:
> 
> test=*# create table foo(u1 int,u2 int);
> CREATE TABLE
> test=*# create unique index idx_foo on foo(least(u1,u2),greatest(u1,u2));
> CREATE INDEX
> 
> 
> But is there a way to define the unique constraint within the create table - command?

You can use exclusion constraints for this.

CREATE TABLE foo (
    u1 integer,
    u2 integer,
    EXCLUDE USING btree (
        least(u1, u2) WITH =,
        greatest(u1, u2) WITH =)
);
-- 
Vik Fearing                                          +33 6 46 75 15 36
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support


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