Question

Here’s some background info. I have three MySQL tables (all InnoDB). The first table is used to store images records. The second table is used to store tags (to tag the images with. OMG). The third table is used to store relationships between the images and the tags. Yippee. Here’s the structure of the tables.

CREATE TABLE `images` (
  `id` int(8) unsigned NOT NULL AUTO_INCREMENT,
  `image` varchar(32) NOT NULL,
  `type` varchar(10) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

CREATE TABLE `tags` (
  `id` int(8) unsigned NOT NULL AUTO_INCREMENT,
  `tag` varchar(64) NOT NULL,
  PRIMARY KEY (`id`) 
) ENGINE=InnoDB

CREATE TABLE `images_to_tags` (
  `image_id` int(8) unsigned NOT NULL,
  `tag_id` int(8) unsigned NOT NULL,
  PRIMARY KEY (`image_id`,`tag_id`),
  KEY `tag_id` (`tag_id`),
  CONSTRAINT `images_to_tags_ibfk_2` FOREIGN KEY (`tag_id`) REFERENCES `tags` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `images_to_tags_ibfk_1` FOREIGN KEY (`image_id`) REFERENCES `images` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB

The type field in the images database is either “image/gif”, “image/png”, or “image/jpeg”.

And there you have it. So my tables are happily populated with a quarter million images, 10,000 tags, and a metric shit load of image-tag relationships.

1) I need to be able to count how many images have tags. Currently I am doing so with the following query:

SELECT COUNT(DISTINCT image_id) 
  FROM images_to_tags

Is this the most efficient way to do this? It seems kind of circuitous. Is this the only reasonable query which will achieve this purpose?

2) For each type of image, I want to find out how many images are tagged. So say about 5,000 total images have tags, how would I know how many of those 5,000 tagged images are of type “image/png”. This isn’t working for me:

SELECT COUNT(id), type 
  FROM images, 
       images_to_tags 
 WHERE images.id = images_to_tags.image_id 

Answer 1

A variation on your first query would be:

select count(*) 
from images_to_tags
group by image_id

For your second:

select type, count(*) 
from images 
where id in (select image_id from images_to_tags)
group by type