/**
  * Returns result of a count(*) query on the given AQL
  * using sql_count select type
  * @param   mixed   $aql    string | aql array
  * @param   array   $clause
  * @return  int
  */
 public static function count($aql, $clause = array())
 {
     $rs = aql::sql_result(self::sql($aql, $clause), array('select_type' => 'sql_count'));
     return $rs[0]['count'];
 }