public function forShare($table_name = null, $glue = ',')
    {
        if (is_null($this->forShare)) {
            $glue = strtoupper($glue);
            $this->forShare = new QueryElement('FOR SHARE', $table_name ? 'OF ' . $table_name : null, "$glue ");
        } else {
            $this->forShare->append($table_name);
        }

        return $this;
    }