EXist

EXist is a native XML database that is distributed as free software under the GNU Lesser General Public License. eXist runs within a Java Virtual Machine and is therefore completely independent of the operating system used.

Data are represented in XML as a structured tree (not common in table form, such as in relational databases). Within this tree you can with XQuery, move the query language for XML documents. eXist stores an XML document internally as a B - tree.