Subject: Re: unique constraint definition within create table
To: Andreas Kretschmer <akretschmer@spamfence.net>, pgsql-sql@postgresql.org
References: <20151202063621.GA5526@tux>
From: Adrian Klaver <adrian.klaver@aklaver.com>
Message-ID: <565F076E.20608@aklaver.com>
Date: Wed, 2 Dec 2015 06:59:58 -0800
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On 12/01/2015 10:36 PM, 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?

http://www.postgresql.org/docs/9.4/interactive/sql-createtable.html

Shows that expressions are not allowed in UNIQUE constraints.

"UNIQUE ( column_name [, ... ] ) index_parameters

whereas

http://www.postgresql.org/docs/9.4/interactive/sql-createindex.html

"CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ name ] ON table_name [ USING 
method ]
     ( { column_name | ( expression ) } ..."

does.

So no there is not a way to do that in the CREATE TABLE command. You can 
bundle the commands though:

BEGIN;
create table foo(u1 int,u2 int);
create unique index idx_foo on foo(least(u1,u2),greatest(u1,u2));
COMMIT;

to make sure they either both succeed or fail.

>
> Thx.
>
> Andreas
>


-- 
Adrian Klaver
adrian.klaver@aklaver.com


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