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
---------------------------(end of broadcast)---------------------------
TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
