Polypheny is a multi-model DBMS built on top of HSQLDB. The DBMS supports the relational, document and Labeled-Property Graph data models. It also supports multiple query interfaces and languages, SQL and Cypher. The data maintained according to different data models can be combined (e.g., joined) within one query.
Relational Document / XML Graph
Polypheny supports their proprietary dialect of SQL called "PolySQL", as well as with compatibility with MongoDB's query language.
N-ary Storage Model (Row/Record)
Polypheny GmbH, University of Basel
Academic, Commercial, Open Source