In this paper, we propose a mapping scheme of XML documents into relational databases. The scheme enables us to store, retrieve and update XML documents efficiently. When storing XML documents in relational databases, XML tree structures must be preserved explicitly. To this end, a label is assigned to nodes in the XML tree. In general, document retrieval and update performance is affected by node labeling schemes. We use SPIDER (Schema based Path IDentifiER), a labeling scheme for XML documents utilizing DTDs that makes retrieval and update more efficient. SPIDER only identifies paths from root node to a node. Thus, multiple nodes appearing in the same path cannot be distinguished by only using SPIDER. We introduced Sibling Dewey Order to identify such nodes. Generally, when a new node is inserted into XML documents, some other nodes need to be relabeled to preserve the order of nodes. In our method, only Sibling Dewey Order is relabeled; SPIDER is not affected. Since the range of relabeling is small, it is possible to update documents efficiently. We stored documents utilizing SPIDER in a relational database and then translated various XPath expressions into SQL using SPIDER. We perform experiments and demonstrate that the proposed scheme outpeforms conventional methods both in retrieval and update.