is a JDBC type 4 driver that allows SQL queries and other JDBC operations on XML files. With the commercial StelsXML JDBC driver, you can access data contained in XML documents by using XPath, and largely standard SQL syntax and features, including joins.
StelsXML has a thirty-day trial version
available; the commercial license costs from $120 USD (two-month technical support and five concurrent connections) to $450 USD for a custom development license (unlimited internal use). A commercial license is also available, cost unspecified, which allows inclusion of the product for resale.
The Java Content Repository specification
provides for something similar to this, in that level II support allows SQL querying of a repository. Since a repository has a hierarchical structure and can easily be seen as an XML document (in theory if not in practice), it requires some of the same basic features provided by StelsXML.
However, importing an XML document into JCR might be a bit of a heavy process, just to use SQL queries over a repository workspace.
What do you think? Could you use the ability to use (most of) SQL on an XML document?