protected function _renderGroup($sql)
    {
        if ($this->_parts[self::FROM] && $this->_parts[self::GROUP]) {
            $group = array();
            foreach ($this->_parts[self::GROUP] as $term) {
                $group[] = $this->_adapter->quoteIdentifier($term, true);
            }
            $sql .= ' ' . self::SQL_GROUP_BY . ' ' . implode(",\n\t", $group);
        }

        return $sql;
    }