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DISCLAIMER: this post is older than one year and may not be up to date with latest WordPress version.
Do you ever wanted to be able to get tags related to one (or more) specific category? If yes, I’m pretty sure you’ll be delighted with this very cool tip.

First, here is the function you have to paste in your function.php file:

function get_category_tags($args) {
	global $wpdb;
	$tags = $wpdb->get_results
	("
		SELECT DISTINCT terms2.term_id as tag_id, terms2.name as tag_name, null as tag_link
		FROM
			wp_posts as p1
			LEFT JOIN wp_term_relationships as r1 ON p1.ID = r1.object_ID
			LEFT JOIN wp_term_taxonomy as t1 ON r1.term_taxonomy_id = t1.term_taxonomy_id
			LEFT JOIN wp_terms as terms1 ON t1.term_id = terms1.term_id,

			wp_posts as p2
			LEFT JOIN wp_term_relationships as r2 ON p2.ID = r2.object_ID
			LEFT JOIN wp_term_taxonomy as t2 ON r2.term_taxonomy_id = t2.term_taxonomy_id
			LEFT JOIN wp_terms as terms2 ON t2.term_id = terms2.term_id
		WHERE
			t1.taxonomy = 'category' AND p1.post_status = 'publish' AND terms1.term_id IN (".$args['categories'].") AND
			t2.taxonomy = 'post_tag' AND p2.post_status = 'publish'
			AND p1.ID = p2.ID
		ORDER by tag_name
	");
	$count = 0;
	foreach ($tags as $tag) {
		$tags[$count]->tag_link = get_tag_link($tag->tag_id);
		$count++;
	}
	return $tags;
}
Once you have pasted the function, you can use it in your theme:

$args = array('categories' => '12,13,14');
$tags = get_category_tags($args);