From: Richard Huxton <dev@archonet.com>
To: Dave Dribin <dave-ml@dribin.org>, pgsql-sql@postgresql.org
Subject: Re: One to many query question
Date: Wed, 30 Jul 2003 22:19:16 +0100
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On Wednesday 30 July 2003 20:35, Dave Dribin wrote:
> Hi, I'm having trouble with what I think should be an easy query.  For
> simplicity, I will use a CD database as an example.  Each CD may have
> multiple genres.  Here's some sample data:
>
> Artist                  Title                           Genres
> ----------------------  ------------------------------  ----------------
> Miles Davis		Some Kind of Blue		Jazz
> Metallica		Ride the Lightning		Rock
> Chemical Brothers	Surrender			Electronic
> Radiohead		OK Computer			Rock, Electronic
>
> For simplicities sake, let's ignore normalization on artist and genre,
> and say the tables look like:
>
> CREATE TABLE cd (
> 	id integer unique,
> 	artist varchar(25),
> 	title varchar(25)
> );
>
> CREATE TABLE cd_genres (
> 	cd_id integer,
> 	genre varchar(25)
> );
>
> How do I write a query to find all CDs that are NOT Rock?  A co-worker
> showed me the following query:

Basically you need to find all the CDs that ARE "rock" and subtract that set 
from the set of all CDs.

You could use:

SELECT id,artist,title FROM cd WHERE NOT EXIST 
(SELECT 1 FROM cd_genres WHERE cd_id=id AND genre='Rock');

or

SELECT id,artist,title,cd_id
FROM cd
LEFT JOIN 
  (SELECT cd_id FROM cd_genres WHERE genre='Rock') AS rock_cds 
ON cd_id=id
WHERE cd_id IS NULL;

or an EXCEPT clause etc.

See which you like better.

-- 
  Richard Huxton
  Archonet Ltd