If the current Web is to evolve into a Semantic Web, eventually
RDB information has to be transported into semantically aware data
structures. To this end, many ontology schemata have been created
and many more are to be defined, a process which requires intense
experimentation with real data. These requirements have led to
the creation of systems for instantiating ontology schemata using
relational database information, with some, like D2RQ and Virtuoso,
gaining wider acceptance in the Semantic Web community.
Unfortunately, all these tools either require advanced user skills as
they are built upon complex declarative languages and lack friendly
user interfaces (thus requiring a hard learning curve), or in case
they support friendly interfaces, they provide limited expressivity,
and/or come with a commercial licence.