Hi list,

I have a many-to-many relationship between movies and genres and, in the
link table I have a third field called which orders the "appropriateness" of
the relations within each movie.

For example:

movie_id, genre_id, relevance (i've used movie/genre titles for clarity
here, but in reality they're id's)
--------------------------------------------
beverly hills cop, action, 2
beverly hills cop, comedy, 1
the heartbreak kid, comedy, 2
the heartbreak kid, romance, 1

The above would mean, to my application:
"Beverly Hills Cop is both an Action and a Comedy movie but primarily an
Action movie."
"The Heartbreak Kid is both a Comedy and a Romance movie but primarily a
Comedy movie."

First of all, if there's a better way to model this kind of ranking/ordering
of many-to-many relationships, please let me know.

Now, to my problem..

I'm taking a subset of all my genres, and I want to get ONE row for each
movie in the subset alongside its most appropriate genre (whichever has the
highest relevance). In other words, the best fit.

I've had a few goes at writing this query but I can't seem to get it right..
The theory in my mind is to:

1. filter the link table down to rows that fit the subset of categories
(easy)

2. filter the link table further to keep only the max(relevance) for each
movie_id .. this is where i'm having trouble.

If someone can shed some light on this for me, I'd really appreciate it.

Thanks for your time,
Jamie

On 10/16/07, Jamie Tufnell <diesql@googlemail.com> wrote:
> I'm taking a subset of all my genres, and I want to get ONE row for each
> movie in the subset alongside its most appropriate genre (whichever has the
> highest relevance). In other words, the best fit.

You didn't provide the expected output, but try:

SELECT *
FROM t tt
WHERE relevance = (SELECT MAX(relevance)
FROM t
WHERE movie_id = tt.movie_id)

---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster

--- Jamie Tufnell <diesql@googlemail.com> wrote:
> movie_id, genre_id, relevance (i've used movie/genre titles for clarity
> here, but in reality they're id's)
> --------------------------------------------
> beverly hills cop, action, 2
> beverly hills cop, comedy, 1
> the heartbreak kid, comedy, 2
> the heartbreak kid, romance, 1
> First of all, if there's a better way to model this kind of ranking/ordering
> of many-to-many relationships, please let me know.

Joe Celko had an example like this in his book:

17: EMPLOYMENT AGENCY PUZZLE
http://www.elsevier.com/wps/find/boo...on#description

the only difference was that he was modeling employees and skillsets. IIRC, the terminology for
the improved model was the "full disjuctive" model.
Regards,
Richard Broersma Jr.


---------------------------(end of broadcast)---------------------------
TIP 3: Have you checked our extensive FAQ?

http://www.postgresql.org/docs/faq

On Oct 16, 2007, at 12:14 , Richard Broersma Jr wrote:

> the only difference was that he was modeling employees and
> skillsets. IIRC, the terminology for
> the improved model was the "full disjuctive" model.

Off chance, might the full disjunction work be relevant here?

http://pgfoundry.org/projects/fulldisjunction/

Michael Glaesemann
grzm seespotcode net



---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster

In article <b0a4f3350710160946y4a31a227sbffbf085cdbfa487@mail .gmail.com>,
"Jamie Tufnell" <diesql@googlemail.com> writes:

> Hi list,
> I have a many-to-many relationship between movies and genres and, in the link
> table I have a third field called which orders the "appropriateness" of the
> relations within each movie.

> For example:

> movie_id, genre_id, relevance (i've used movie/genre titles for clarity here,
> but in reality they're id's)
> --------------------------------------------
> beverly hills cop, action, 2
> beverly hills cop, comedy, 1
> the heartbreak kid, comedy, 2
> the heartbreak kid, romance, 1

> The above would mean, to my application:
> "Beverly Hills Cop is both an Action and a Comedy movie but primarily an Action
> movie."
> "The Heartbreak Kid is both a Comedy and a Romance movie but primarily a Comedy
> movie."

> First of all, if there's a better way to model this kind of ranking/ordering of
> many-to-many relationships, please let me know.

This looks fine to me.

> Now, to my problem..

> I'm taking a subset of all my genres, and I want to get ONE row for each movie
> in the subset alongside its most appropriate genre (whichever has the highest
> relevance). In other words, the best fit.

You could use something like that:

SELECT m.name, g.name, mg.relevance
FROM movies m
JOIN mg ON mg.movie_id = m.id
JOIN genres g ON g.id = mg.genre_id
LEFT JOIN mg mg1 ON mg1.movie_id = mg.movie_id
AND mg1.relevance > mg.relevance
WHERE mg1.movie_id IS NULL

This means that there must not be an link table entry for the same
movie with a higher relevance.


---------------------------(end of broadcast)---------------------------
TIP 6: explain analyze is your friend