One of the things that I think tends towards being a bit of an elephant in the Linked Data room is the practically difficulty of running a query that links together results from two different datastores, even if they share common identifiers. The solution – at the moment at least – seems to require grabbing a dump of both datastores, uploading them to a common datastore and then querying that…
…which means you need to run your own triple store…
This quick post links out the the work of two others, as much as a placeholder for myself as for anything, describing how to get started doing exactly that…
First up, John Goodwin, aka @gothwin, (a go to person if you ever have dealings with the Ordnance Survey Linked Data) on How can I use the Ordnance Survey Linked Data: a python rdflib example. As John describes it:
[T]his post shows how you just need rdflib and Python to build a simple linked data mashup – no separate triplestore is required! RDF is loaded into a Graph. Triples in this Graph reference postcode URIs. These URIs are de-referenced and the RDF behind them is loaded into the Graph. We have now enhanced the data in the Graph with local authority area information. So as well as knowing the postcode of the organisations taking part in certain projects we now also know which local authority area they are in. Job done! We can now analyse funding data at the level of postcode, local authority area and (as an exercise for the ready) European region.
Secondly, if you want to run a fully blown triple store on your own localhost, check out this post from Jeni Tennison, aka @jenit, (a go to person if you’re using the data.gov.uk Linked Datastores, or have an interest in the Linked Data JSON API): Getting Started with RDF and SPARQL Using 4store and RDF.rb, which documents how to get started on the following challenges (via Richard Pope’s Linked Data/RDF/SPARQL Documentation Challenge):
Install an RDF store from a package management system on a computer running either Apple’s OSX or Ubuntu Desktop.
Install a code library (again from a package management system) for talking to the RDF store in either PHP, Ruby or Python.
Programatically load some real-world data into the RDF datastore using either PHP, Ruby or Python.
Programatically retrieve data from the datastore with SPARQL using using either PHP, Ruby or Python.
Convert retrieved data into an object or datatype that can be used by the chosen programming language (e.g. a Python dictionary).
PS it may also be worth checking out these posts from Kingsley Idehen:
– SPARQL Guide for the PHP Developer
– SPARQL Guide for Python Developer
– SPARQL for the Ruby Developer