Re: tricky CHECK condition

Multi-column unique index?

> How could I specify a condition in 'CHECK' constraint for a column to
> check that its value must only appear once in *some* rows of a table?
> Sort of a local "UNIQUE"... For example, in CDs world:
>
> create table tracks (
> -- Unique track identifier
> id serial primary key,
> -- Which disk the track is from
> disk_id int4 references disks (id),
> -- Track number in a disk
> track_number int2,
> check (WHAT I WANT GOES HERE) );
>
> I want to check values of `track_number' to be unique for the
> disk. Basically, I want to check that an SQL query
>
> select track_number from tracks T
> where T.disk_id=disk_id and T.track_number=track_number;
>
> is empty... Is it possible to run an SQL query in a condition clause?
> I looked thru documentation, but couldn't find anything relevant.
> --
> Arcady Genkin http://www.thpoon.com
> Nostalgia isn't what it used to be.
>

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