$this->db->select('word')->from('words')->where('wordID', 3);
$sub = $this->subquery->start_subquery('select');
$sub->select('number')->from('numbers')->where('numberID', 2);

$sub2 = $this->subquery->start_subquery('where_in');
$sub2->select('test')->from('testing')->where('a', 12);
$this->subquery->end_subquery('ab');

$this->subquery->end_subquery('number');