I am doing (want to do) some experiments with Linked Open Datasets particularly those put out by governments.

I have a RDBMS (more specifically MySQL). I designed it with semantic web ideas in mind i.e. I have a information stored as objects, predicates and classes which define objects. In turn all objects are related to each other though statements of the form subject --> predicate --> object (where the subjects are from the objects table).

I want to be able to query other RDF triple stores from my application and let other triple stores query my data. Is it possible to "set something up" so that this is possible?

I have looked at Jena. Using Jena seems to mean I have to it as a storage application rather than MySQL - the only problem with this is that I include a new concept called a category (which I don't think is part of the semantic web languages). I will use categories to help with displaying information (they don't have any other meaning) but using Jena seems to mean that I can't organise predicates under categories for more convenient viewing.

I am using Java so a JAVA API is preferred.

It's also possible I misunderstood the purpose of Jena, and maybe that can be of use, but I am not sure how.

I am sure four days from now this question will seem rather silly, but at the moment I am somewhat confused about how to proceed.