Referential integrity with primary key spanning multiple columns?

On Fri, 2 Aug 2002, Jean-Christian Imbeault wrote:

...

-- GENRES

create table GENRES (

major_genre_id int2 not null,
minor_genre_id int2 not null,
genre_desc text not null,

primary key (major_genre_id, minor_genre_id)
);

-- REL_GENRES_MOVIES

create table REL_GENRES_MOVIES (

prod_id integer references MOVIES,
major_genre_id int2 references GENRES(major_genre_id),
minor_genre_id int2 references GENRES(minor_genre_id),

primary key (prod_id, major_genre_id, minor_genre_id)
);

(The error is for this last table REL_GENRES_MOVIES)

NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
'rel_genres_movies_pkey' for table 'rel_genres_movies'
NOTICE: CREATE TABLE will create implicit trigger(s) for FOREIGN KEY
check(s)
ERROR: UNIQUE constraint matching given keys for referenced table
"genres" not found

Does this mean that I cannot have a foreign key unless it is a UNIQUE
field by itself? I was hoping that since major_genre and minor_genre
together are unique I could use them as foreign keys ...

I believe the referenced column must be unique as you suggest.

If the problem is that neither is unique by itself can anyone recommend
some other way where I can use referential integrity with those two fields?

You could try using:

create table REL_GENRES_MOVIES (

prod_id integer references MOVIES,
major_genre_id int2 ,
minor_genre_id int2 ,

primary key (prod_id, major_genre_id, minor_genre_id),

foreign key (major_genre_id, minor_genre_id) references genres(major_genre_id,
minor_genre_id)
);

although I've never tried it so don't know if that really does what you want
but it looks like it should.

--
Nigel J. Andrews
Director

---
Logictree Systems Limited
Computer Consultants