Tags
—-
Tag
PostID

Posts
—-
PostID
Post

QUERY
—–
SELECT * FROM Posts WHERE PostID IN (SELECT PostID FROM Tags WHERE Tag = ‘$tag1′ OR Tag = ‘$tag2′)

Big Upz to the MySQL wiki for the tips – ironic huh?
http://forge.mysql.com/wiki/TagSchema#Toxi

CREATE TABLE links (
l_id SERIAL,
title varchar(55),
url varchar(255),
description varchar(300),
CONSTRAINT “links_pkey” PRIMARY KEY(“l_id”)
);

CREATE TABLE tmap (
l_id integer REFERENCES links ON DELETE CASCADE,
t_id integer REFERENCES tags,
PRIMARY KEY (l_id, t_id)
);

CREATE INDEX tag_id_idx ON tmap (t_id);

CREATE TABLE tags (
t_id SERIAL,
tag varchar(30) UNIQUE,
CONSTRAINT “tags_pkey” PRIMARY KEY(“t_id”)
);

/////////////////////////////////

// grab data for a TAG cloud
SELECT tag, COUNT(*) as count
FROM tmap i2t
INNER JOIN tags t
ON i2t.t_id = t.t_id
GROUP BY tag;

// grab data for links tagged with any of the 3 tags
SELECT tmap.l_id
FROM tmap
INNER JOIN tags t
ON tmap.t_id = t.t_id
WHERE t.tag IN (‘transcripts’,'registration’,'courses’)
GROUP BY tmap.l_id;

// grab data for links tagged with all three tags being searched for
SELECT i2t3.l_id
FROM tags t1 CROSS JOIN tags t2 CROSS JOIN tags t3
INNER JOIN tmap i2t1
ON t1.t_id = i2t1.t_id
INNER JOIN tmap i2t2
ON i2t1.l_id = i2t2.l_id AND i2t2.t_id = t2.t_id
INNER JOIN tmap i2t3
ON i2t2.l_id = i2t3.l_id AND i2t3.t_id = t3.t_id
WHERE t1.tag = ‘transcripts’
AND t2.tag = ‘registration’
AND t3.tag = ‘courses’;

http://www.miskatonic.org/library/facet-web-howto.html

I hope someone else finds it useful.

I never even considered anything other than a relational 3 table schema. The system will be 99% selects so I suppose that’s best anyway:
I’m making an e-commerce site.. Tags would only be added when new products were added (no user contributions).

I’m considering using tags rather than categories because the categories aren’t forming a nice hierarchy.
Now I’m going to look into hierarchical tags systems.

THANKS!

One thing I did notice that you might care about is your disadvantage for Toxi’s schema;
“When altering or deleting bookmarks you can end up with tag-orphans.”
This isn’t the case if you use foreign keys (which Phillip Keller implements above), and if you add ON DELETE CASCADE to your reference clause any deletion of tags or items will also delete references to it in the intersection table- for better or worse.