$this->db->where('user_status', NULL);
$query_1 = $this->db->get_compiled_select('users');
$this->db->where('user_status', 'inactive');
$query_2 = $this->db->get_compiled_select('users');
$query_union = $this->db->query($query_1 . " UNION " . $query_2);
https://stackoverflow.com/questions/2040655/union-query-with-codeigniters-active-record-pattern