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;
    }

wp_是表前缀，看需要改吧。
下边是调用方法：

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

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