Question

If anyone could recommend a good book for learning mySQL as well, that would be great :).

I have two tables, tags, codes_tags

CREATE TABLE `tags` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `name` varchar(40) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL,
 PRIMARY KEY (`id`),
 UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=190 DEFAULT CHARSET=utf8


CREATE TABLE `codes_tags` (
 `code_id` int(11) unsigned NOT NULL,
 `tag_id` int(11) unsigned NOT NULL,
 KEY `sourcecode_id` (`code_id`),
 KEY `tag_id` (`tag_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

What I am trying to do is select the name from ‘tags’, and how many of that tag_id there are in ‘codes_tags’, and order them by that count. If there is no records in codes_tags for that tag_id, ‘count’ should be equal to 0 or NULL (preferably 0).

This is the closest I have come so far:

SELECT tags.name, COUNT( codes_tags.tag_id ) AS count
FROM tags
LEFT JOIN codes_tags ON tags.id = codes_tags.tag_id
GROUP BY tag_id
ORDER BY count DESC
LIMIT 0 , 30

It seems to do what I am wanting, however it is only returning four rows when it should return 30.

What am I doing wrong here?
Thanks.

Answer 1

I think if you change your COUNT(codes_tags.tag_id) to COUNT(*) in the SELECT, that NULLs will also be included. (If it’s nulls or 0 counts that you’re missing. Otherwise, the query looks fine).

EDIT: On second thought, I missed the LEFT JOIN. That would mean you want all of the tags even if they’re not related to something in the codes_tags table. Is that what you want?

I would probably do something like the following:

SELECT tags.name, COUNT(*) AS count
FROM tags
INNER JOIN codes_tags ON tags.id = codes_tags.tag_id
GROUP BY tags.id
ORDER BY count(*) DESC

It can be inferred from the items not in the list which tags are not also included in codes_tags. However, if you wanted to explicitly do that as well:

SELECT tags.name, COUNT(*) AS count
FROM tags
INNER JOIN codes_tags ON tags.id = codes_tags.tag_id
GROUP BY tags.id
UNION
SELECT tags.name, '0'
from tags
where tags.name not in 
   (SELECT tags.name
   FROM tags
   INNER JOIN codes_tags ON tags.id = codes_tags.tag_id)
ORDER BY count(*) DESC

(I don’t have access to a SQL box at the moment, so take the queries with a grain of salt; they’re untested.)