SPARQL

SPARQL is the standard query language used to extract information from RDF data. In rdfpub, SPARQL queries are used to extract information about your resources for use in Handlebars templates.

Creating SPARQL queries

A SPARQL query is created for each .sparql file in your site directory. The 'name' part of the file (the file name minus the .sparql extension) is used to reference the results of the query in Handlebars templates.

Query inheritance

When an rdfpub site is built, SPARQL queries are recursively copied down from its own directory to all subdirectories. This allows you to write a generic SPARQL query that all 'child' resources can use without having to duplicate your queries for every resource that needs them.

For example, the lesson query of the tutorial site is passed down to each of the individual tutorial lessons.

Generic queries

In order to make queries generic, rdfpub will run your query against every resource that inherits it with two special variables:

• $resource - the resource that the query is being executed for
• $language - the current language that is currently being queried for (explained in greater detail in the i18n lesson)

With these two variables, you can write queries that can apply to any resource that they're run for. For instance, let's say that you want to write a SPARQL query that extracts a name and description for a person. Such a query might look like this:

SELECT $name $description
WHERE {
  GRAPH $resource {
    $resource a foaf:Person
    ; foaf:name $name
    ; dcterms:description $description
  }
}

If you define this query as person.sparql in a directory /person, then every resource in a subdirectory will receive this query with its own values injected into it. So /person/Alice and /person/Bob would have their own set of results for this query based on their individual URL's and RDF data.

Again using the lesson query as an example, not only does it get passed down to each tutorial lesson, but it also get run against each lesson with its $resource variable bound to each lesson's URL and its $language variable bound to each language that the lesson supports.

Supported SPARQL query types

As of this writing, only SPARQL tuple queries are supported, that is, queries that return SPARQL results created from a SELECT query. All of ASK, CONSTRUCT, and DESCRIBE are intended to be supported in the future. For now, SELECT queries should suffice for almost all practical purposes.

SPARQL endpoint

The rdfpub site generator creates a SPARQL endpoint for every site that it generates. This endpoint can be used to make arbitrary SPARQL queries over your data. This can be useful for anyone looking to examine or analyze your data. It also allows others to federate their own SPARQL queries to your site by way of the SERVICE SPARQL keyword, thus connecting datasets across the web.

As is common convention, an rdfpub site's SPARQL endpoint is served from the /sparql URL by default. It is a SPARQL 1.1 compliant endpoint except that it does not support SPARQL update operations as rdfpub sites are read-only. A site's SPARQL endpoint also serves SPARQL 1.1 service description data which describes the SPARQL endpoint and the features/data it supports.

Lastly, you can publish an HTML page for your SPARQL endpoint by putting an index.handlebars file in your SPARQL endpoint directory. This tutorial site does exactly that with its own SPARQL endpoint. Index templates are explained in the next lesson.

If you want to see a live query in action, you can try it for yourself with this query to fetch lessons from this tutorial site's SPARQL endpoint.