We propose a uniform and flexible mechanism to make reference links from SGML documents to database objects. In addition to typical document logical structures such as sections and paragraphs, our mechanism allows arbitrary character strings in documents as source of these links. By using this mechanism, SGML attributes and their values of marked-up words can be transparently stored as database attributes, and we can establish hyperlinks between keywords in documents, which reflect relationships between the corresponding database objects. Also, we present a query language to retrieve SGML documents which are coupled with databases in this manner. The query language does not assume a particular database schema; instead, it utilizes DTD graphs, representing element structures of DTDs, as virtual schemas.