MySQL select matching ids, output to json

Question

I'm using d3js on a visual display of user-generated content. Data that is being displayed is being pulled from the database with the standard three tables: entry, entry tags, entry-tag look-up table. My d3js code needs JSON output for this to work. I have the PHP-to-JSON output coded correctly, but am having a hard time figuring out what query I need to write to pull the correct IDs.

Entry table (create_thought)

id | email | thought_title

Tags table (thoughts_tags)

tag_id | tag_name

Look-up table (thoughts_tagged)

tagged_id | tag_id | thought_id

PHP-to-JSON code (functioning)

$titlequery = "
SELECT  `thought_title` FROM  `create_thought`
";
//THE BELOW QUERY IS INCORRECT
$tagquery = "
SELECT  `tag_id` AS `source`, `thought_id` AS `target` FROM  `thoughts_tagged`
";
$query1 = mysql_query($titlequery);

if ( ! $query1 ) {
echo mysql_error();
die;
}
$query2 = mysql_query($tagquery);

if ( ! $query2 ) {
echo mysql_error();
die;
}

$datatitle = array();

for ($x = 0; $x < mysql_num_rows($query1); $x++) {
$datatitle[] = mysql_fetch_assoc($query1);
}

$datatag = array();

for ($y = 0; $y < mysql_num_rows($query2); $y++) {
$datatag[] = mysql_fetch_assoc($query2);
}

echo '{
"nodes":' . json_encode($datatitle) . ',"links":' . json_encode($datatag) . '}';     

This outputs to something like:

{ "nodes":[{"thought_title":"Title1"},{"thought_title":"Title2"}],"links":[{"source":"1","target":"8"},{"source":"2","target":"8"},{"source":"2","target":"17"}]}

My problem is the links (source/target) are all incorrect. I need the source to be the thought_id and the target to be all thought_ids with matching tag_ids. I've tried fiddling with some of the solutions from How to filter SQL results in a has-many-through relation , but they aren't what I need.

The below is the closest that I've gotten, but I'm not sure how to format it with the source and target; it's just pulling the thought_id with the one matching tag (55).

SELECT id
FROM   create_thought
WHERE  EXISTS (SELECT * FROM thoughts_tagged
WHERE  thought_id = id AND tag_id = 55)

Edit: Came up with this, which seems to be closer, but it is not formatted correctly. I need the thought_ids listed based on matching tags, but nothing else.

SELECT DISTINCT t1.*
FROM thoughts_tagged t1
INNER JOIN thoughts_tagged t2
ON t1.tag_id = t2.tag_id AND t1.thought_id <> t2.thought_id

Any help is appreciated!

Answer 1

Here was the answer a developer I know was able to come up with. It works perfect. :)

SELECT DISTINCT
t1.thought_id as source,
t2.thought_id as target
FROM thoughts_tagged t1
INNER JOIN
thoughts_tagged t2
ON t1.tag_id = t2.tag_id AND t1.thought_id <> t2.thought_id