Best practice for storing tags in a database?

Question

I developed a site that uses tags (key words) in order to categorize photographs. Right now, what I have in my MySQL database is a table with the following structure:

Answer 1

In multi tag search query you will have to hit every tag that is requested. Hence image tag set I has to be a superset of the request tag set U.

I >= U

To implement this complex comparison in SQL is a bit of challenge as each of the image has to be qualified individually. Given that tags are unique set per image:

SELECT i.* FROM images AS i WHERE {n} = (
  SELECT COUNT(*) 
  FROM image_tags AS t 
  WHERE t.image_id = i.image_id
    AND t.tag IN ({tag1}, {tag2}, ... {tagn})
)

Schema:

CREATE TABLE images (
  image_id varchar NOT NULL,
  PRIMARY KEY (image_id)
)

CREATE TABLE image_tags (
  image_id varchar NOT NULL,
  tag varchar NOT NULL,
  PRIMARY KEY (image_id, tag)
)