public function getSql()
    {
        if ($this->adapter === null) {
            $msg = __METHOD__ . ": Error, prior to use execute method you must provide a valid database adapter. See Select::setDbAdapter() method.";
            throw new Exception\InvalidUsageException($msg);
        }
        $sql = new Sql($this->adapter);
        return $sql->getSqlStringForSqlObject($this);
    }