Ashpool

If you're looking for a free and open source Java-based lightweight XML database software, look no further than Ashpool. This software is designed to use SQL92 syntax in order to query, add, update, and delete XML documents via its JDBC driver.

One important thing to note about Ashpool is that it requires XML documents to adhere to a table-like structure. This is because SQL was designed specifically to query table data, rather than hierarchical data. However, the Ashpool engine via JDBC drivers can be used to query any XML document, but it is most efficient when working with documents that have a table-like structure for their first three levels.

The "column" section of this table structure can contain a variety of data types including integer, float, string, dateTime, or XML fragments as well. Ashpool can be used in many ways, such as searching for XML documents within another XML document (e.g. as a serialized bean store) or as a lightweight relational database.

Overall, Ashpool is a powerful and versatile XML database software that could be the perfect solution for your needs.