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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