{"id":699,"date":"2004-10-17T12:23:38","date_gmt":"2004-10-17T12:23:38","guid":{"rendered":"http:\/\/beta.robertprice.co.uk\/robblog\/2004\/10\/querying_rdf_in_perl_with_rdf_core-shtml\/"},"modified":"2004-10-17T12:23:38","modified_gmt":"2004-10-17T12:23:38","slug":"querying_rdf_in_perl_with_rdf_core-shtml","status":"publish","type":"post","link":"https:\/\/www.robertprice.co.uk\/robblog\/querying_rdf_in_perl_with_rdf_core-shtml\/","title":{"rendered":"Querying RDF In Perl With RDF::Core"},"content":{"rendered":"

\nExtracting data from an RDF<\/acronym> file seems like an easy job for Perl<\/a>.\n<\/p>\n

\nI have discussed how to parse RDF in Perl<\/a> using the RDF::Core<\/a> module before. In that example I used the getStmts<\/code> functionality. This is fine for simple things, but it’s a bit messy. Thankfully RDF::Core provides us with a query language similar to RDQL<\/a> to help us get the data we want.\n<\/p>\n

\nLet’s take a quick look at my FOAF<\/acronym> file<\/a> for some example RDF to query. I know Cal Henderson<\/a>, and this is represented in my FOAF<\/acronym> file as…\n<\/p>\n

<foaf:knows>
\n<foaf:Person>
\n<foaf:nick>Cal<\/foaf:nick>
\n<foaf:name>Cal Henderson<\/foaf:name>
\n<foaf:mbox_sha1sum>2971b1c2fd1d4f0e8f99c167cd85d522a614b07b<\/foaf:mbox_sha1sum>
\n<rdfs:seeAlso rdf:resource=\"http:\/\/www.iamcal.com\/foaf.xml\"\/>
\n<\/foaf:Person>
\n<\/foaf:knows>
\n<\/code><\/div>\n

\nFor a detailed explantion of the above RDF, have a look at my previous article – What Is An RDF Triple?<\/a>\n<\/p>\n

\nHow could I find Cal’s foaf file using RDF::Core’s query language? It’s really simple and we’d use the following query.\n<\/p>\n

select ?x->rdfs:seeAlso,
\nfrom ?x->foaf:nick{?y}
\nwhere ?y='Cal'
\n<\/code><\/div>\n

\nThat just means we want to select the value of the triple rdfs:seeAlso where the parent triple has the foaf:nick of the value of Cal.\n<\/p>\n

\nWhy stop there? Lets get some more information on Cal.\n<\/p>\n

select ?x->foaf:name, ?x->foaf:nick, ?x->rdfs:seeAlso, ?x->foaf:mbox_sha1sum
\nfrom ?x->foaf:nick{?y}
\nwhere ?y='Cal'
\n<\/code><\/div>\n

\nNow we have Cal’s name, nickname, address of his FOAF file and the checksum of his email address.\n<\/p>\n

\nLet’s see how to get this all in Perl now.\n<\/p>\n

\nWe need to setup some RDF::Core objects and parse the FOAF file before we can query it. We’ll use this code to do so.\n<\/p>\n

## list of known namespaces we might need.
\nmy $namespaces = {
\n'foaf'\t=> 'http:\/\/xmlns.com\/foaf\/0.1\/',
\n'rdfs'\t=> 'http:\/\/www.w3.org\/2000\/01\/rdf-schema#',
\n'rdf'\t=> 'http:\/\/www.w3.org\/1999\/02\/22-rdf-syntax-ns#',
\n'dc'\t=> 'http:\/\/purl.org\/dc\/elements\/1.1\/',
\n};
\n## create our model and storage for triples.
\nmy $store = RDF::Core::Storage::Memory->new;
\nmy $model = RDF::Core::Model->new(Storage => $store);
\n## create node factory for query
\nmy $factory = RDF::Core::NodeFactory->new;
\n## create our parse and parse the Source file.
\nmy $parser = RDF::Core::Model::Parser->new(
\nModel => $model,
\nBaseURI => 'http:\/\/xmlns.com\/foaf\/0.1\/',
\nSource => '.\/foaf.rdf',
\nSourceType => 'file'
\n);
\n$parser->parse;
\n<\/code><\/div>\n

\nFor a full explanation of the above code, have a look at my previous article – How To Parse RDF In Perl<\/a>.\n<\/p>\n

\nWe now need to add the new code.\n<\/p>\n

\nFirstly we need to a RDF::Core::Evaluator and a RDF::Core::Query object. These two objects handle the querying of the data held in our $model<\/var>.\n<\/p>\n

## create an evaluator based on our model, factory and namespaces.
\nmy $evaluator = RDF::Core::Evaluator->new(
\nModel\t=> $model,
\nFactory => $factory,
\nNamespaces => $namespaces,
\n);
\n## create a query object based on the evaluator.
\nmy $query = RDF::Core::Query->new(Evaluator => $evaluator);
\n<\/code><\/div>\n

\nAs we have a query object, we can just insert our query statement using it’s query method.\n<\/p>\n

## run our query and save the results in $results.
\nmy $results = $query->query(\"select ?x->foaf:name, ?x->foaf:nick, ?x->rdfs:seeAlso, ?x->foaf:mbox_sha1sum from ?x->foaf:nick{?y} where ?y='Cal'\");
\n<\/code><\/div>\n

\nThis executes the query and returns the results as a reference to an array in the $results<\/var> variable. We need to use the first arrayref returned in $results<\/var> to get the data we need. This will be a list of RDF::Core::Literal or RDF::Core::Resource objects. As these both inherit from RDF::Core::Node, let’s just use that object’s getLiteral method to return the string values of the data we need and print it out.\n<\/p>\n

## go over the results and print the data out.
\nforeach my $result (@{$results->[0]}) {
\nprint $result->getLabel, \"n\";
\n}
\n<\/code><\/div>\n

\nThis will return us the following data.\n<\/p>\n

Cal Henderson
\nCal
\nhttp:\/\/www.iamcal.com\/foaf.xml
\n2971b1c2fd1d4f0e8f99c167cd85d522a614b07b
\n<\/code><\/div>\n

\nHere’s the final code in all it’s glory.\n<\/p>\n

#!\/usr\/bin\/perl -w
\nuse strict;
\nuse RDF::Core::Evaluator;
\nuse RDF::Core::Model;
\nuse RDF::Core::Model::Parser;
\nuse RDF::Core::NodeFactory;
\nuse RDF::Core::Query;
\nuse RDF::Core::Resource;
\nuse RDF::Core::Storage::Memory;
\n## list of known namespaces we might need.
\nmy $namespaces = {
\n'foaf'\t=> 'http:\/\/xmlns.com\/foaf\/0.1\/',
\n'rdfs'\t=> 'http:\/\/www.w3.org\/2000\/01\/rdf-schema#',
\n'rdf'\t=> 'http:\/\/www.w3.org\/1999\/02\/22-rdf-syntax-ns#',
\n'dc'\t=> 'http:\/\/purl.org\/dc\/elements\/1.1\/',
\n};
\n## create our model and storage for triples.
\nmy $store = RDF::Core::Storage::Memory->new;
\nmy $model = RDF::Core::Model->new(Storage => $store);
\n## create node factory for query
\nmy $factory = RDF::Core::NodeFactory->new;
\n## create our parse and parse the Source file.
\nmy $parser = RDF::Core::Model::Parser->new(
\nModel => $model,
\nBaseURI => 'http:\/\/xmlns.com\/foaf\/0.1\/',
\nSource => '.\/foaf.rdf',
\nSourceType => 'file'
\n);
\n$parser->parse;
\n## create an evaluator based on our model, factory and namespaces.
\nmy $evaluator = RDF::Core::Evaluator->new(
\nModel\t=> $model,
\nFactory => $factory,
\nNamespaces => $namespaces,
\n);
\n## create a query object based on the evaluator.
\nmy $query = RDF::Core::Query->new(Evaluator => $evaluator);
\n## run our query and save the results in $results.
\nmy $results = $query->query(\"select ?x->foaf:name, ?x->foaf:nick, ?x->rdfs:seeAlso, ?x->foaf:mbox_sha1sum from ?x->foaf:nick{?y} where ?y='Cal'\");
\n## go over the results and print the data out.
\nforeach my $result (@{$results->[0]}) {
\nprint $result->getLabel, \"n\";
\n}
\n<\/code><\/div>\n

\nIf you see warnings (when running using -w<\/code> or use warnings;<\/code>) from RDF::Core about use of an uninitialised value before the results are printed don’t worry. This is just a small bug in RDF::Core and won’t affect the running of the code.<\/p>\n","protected":false},"excerpt":{"rendered":"

Extracting data from an RDF file seems like an easy job for Perl. I have discussed how to parse RDF in Perl using the RDF::Core module before. In that example I used the getStmts functionality. This is fine for simple things, but it’s a bit messy. Thankfully RDF::Core provides us with a query language similar … Continue reading “Querying RDF In Perl With RDF::Core”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[2],"tags":[47,56],"class_list":["post-699","post","type-post","status-publish","format-standard","hentry","category-dev","tag-perl","tag-rdf"],"yoast_head":"\nQuerying RDF In Perl With RDF::Core - Robert Price<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.robertprice.co.uk\/robblog\/querying_rdf_in_perl_with_rdf_core-shtml\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Querying RDF In Perl With RDF::Core - Robert Price\" \/>\n<meta property=\"og:description\" content=\"Extracting data from an RDF file seems like an easy job for Perl. 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