protected function _renderUnion($sql)
    {
        if ($this->_parts[self::UNION]) {
            $parts = count($this->_parts[self::UNION]);
            foreach ($this->_parts[self::UNION] as $cnt => $union) {
                list($target, $type) = $union;
                if ($target instanceof Zend_Db_Select) {
                    $target = $target->assemble();
                }
                $sql .= $target;
                if ($cnt < $parts - 1) {
                    $sql .= ' ' . $type . ' ';
                }
            }
        }

        return $sql;
    }