Habari\Vocabulary::get_root_terms PHP Method

    public function get_root_terms()
    {
        /**
         * If we INNER JOIN the terms table with itself on ALL the descendants,
         * then descendants one level down are listed once, two levels down are listed twice,
         * etc. If we return only those terms which appear once, we get root elements.
         * ORDER BY NULL to avoid the MySQL filesort.
         */
        $query = <<<SQL
SELECT child.term as term,
\tchild.term_display as term_display,
\tchild.mptt_left as mptt_left,
\tchild.mptt_right as mptt_right,
\tchild.vocabulary_id as vocabulary_id,
\tchild.id as id
FROM {terms} as parent
INNER JOIN {terms} as child
\tON child.mptt_left BETWEEN parent.mptt_left AND parent.mptt_right
\tAND child.vocabulary_id = parent.vocabulary_id
WHERE parent.vocabulary_id = ?
GROUP BY child.term
HAVING COUNT(child.term)=1
ORDER BY mptt_left ASC
SQL;
        return new Terms(DB::get_results($query, array($this->id), 'Term'));
    }