What's wrong with this mySQL query?

Question

Consider following two tables:

tag_names (tag_id, tag_name)
tag_links (tag_id, image_id)

An image can have multiple tags, I want to select all tags for a specific image id. I am trying following query, but it doesnt seem to select correctly (selects only one row), What is wrong with it?

SELECT tag_name
FROM tag_names
LEFT JOIN tag_links.tag_id = tag_names.tag_id
WHERE tag_links.image_id = $image_id

Edit: I'm using CodeIgniter Active record query, but I wrote in basic SQL format so that if someone is not fimiliar with CodeIgniter can help. However, this query works fine with simple mysql format (without using CodeIgniter) but strangely does not work with CodeIgniter, even there is no any problem with the syntax, it just selects one row.

Here is CodeIgniter Syntax:

$this->db->select('tag_name');
$this->db->from('tag_names');
$this->db->join('tag_links', 'tag_links.tag_id = tag_names.tag_id', 'left');
$this -> db -> where('tag_links.image_id', (int)$image_id);     
$query = $this->db->get();

Answer 1

Try this:

SELECT tag_name
FROM tag_names
LEFT JOIN tag_links
ON tag_links.tag_id = tag_names.tag_id
WHERE tag_links.image_id = $image_id

IMHO you forgot to join table (properly with ON statement) you are using.

EDIT: I have 2 ideas how to get rid of the problem:

First: Change the line with SELECT

$this->db->select('tag_names.tag_name');

Second: Use select() function with complete query:

$this->db->select($query, false);

$this->db->select() accepts an optional second parameter. If you set it to FALSE, CodeIgniter will not try to protect your field or table names with backticks. This is useful if you need a compound select statement.

from: http://codeigniter.com/user_guide/database/active_record.html#select

Answer 2

It seems that you have a syntax error (you forgot tag_links in JOIN clause). By the way in my opinion you don't need LEFT JOIN for this purpose otherwise you may get incorrect results.

SELECT tag_name
FROM 
    tag_names
    JOIN tag_links ON tag_links.tag_id = tag_names.tag_id
WHERE tag_links.image_id = $image_id

Answer 3

SELECT tag_names.tag_name
FROM tag_links
LEFT JOIN tag_names.tag_id = tag_links.tag_id
WHERE tag_links.image_id = $image_id

tag_names is only going to have single entry for a given ID, which means your query will return a single result. You need to primarily select from tag_links and then join the name of the tag on top of it, so you correctly select from the table with the multiple entries.