Day: July 30, 2009

SPARQL’s str() function in Tracker

Today I implemented the str() function for our SPARQL engine.

This makes it possible to use a <subject> just like a string.

Let’s first insert some data into our SPARQL store.

tracker-sparql -u -q \
   "INSERT { <urn:baaa> a rdfs:Resource }"

Following query doesn’t work, as variable ?s isn’t assigned with a xsd:string here, but a rdfs:Resource.

tracker-sparql -q
"SELECT ?s WHERE {
	?s a rdfs:Resource .
	FILTER REGEX (?s, '.*baaa', 's')
}"

This version works, because we introduce the str() function.

tracker-sparql -q
"SELECT ?s WHERE {
	?s a rdfs:Resource .
	FILTER REGEX (str(?s), '.*baaa', 's')
}"
  urn:uuid:94baaa45-99a6-e0f4-0bd9-f83ca90a9039
  urn:uuid:6e909006-a6ac-baaa-2ae4-cc01adcd5de7
  urn:baaa

You can also use a direct match, of course.

tracker-sparql -q
"SELECT ?s WHERE {
	?s a rdfs:Resource .
	FILTER (str(?s) = 'urn:baaa')
}"
  urn:baaa

By the way. Ivan made a cute tool in Python for typing in your queries:

It even does some code completion. If you type nco:[TAB] it’ll show you the NCO ontology. Nice!