.. _example13: Example 13: SPARQL query forms ------------------------------ SPARQL provides alternatives to the standard SELECT query. This example exercises these alternatives to show how AllegroGraph Server and the Python client handle them. Let's connect to the database: .. literalinclude:: doctest_setup.py :language: python_rdf :start-after: BEGIN-CONNECT :end-before: END-CONNECT We'll need some sample data to illustrate all the query types. Our dataset will contain information about rulers of 17th century England. .. testcode:: example13 conn.addData(""" @prefix : . @prefix xsd: . :james_i :reigned_from "1603-03-24"^^xsd:date ; :reigned_to "1625-03-27"^^xsd:date . :charles_i :reigned_from "1625-03-27"^^xsd:date ; :reigned_to "1649-01-30"^^xsd:date ; :child_of :james_i . :charles_ii :reigned_from "1649-01-30"^^xsd:date ; :reigned_to "1685-02-06"^^xsd:date ; :child_of :charles_i . :james_ii :reigned_from "1685-02-06"^^xsd:date ; :reigned_to "1688-12-11"^^xsd:date ; :child_of :charles_i . :mary_ii :reigned_from "1689-02-13"^^xsd:date ; :reigned_to "1694-12-28"^^xsd:date ; :child_of :james_ii . :william_iii :reigned_from "1672-07-04"^^xsd:date ; :reigned_to "1702-03-08"^^xsd:date . :anne :reigned_from "1707-05-01"^^xsd:date ; :reigned_to "1714-08-01"^^xsd:date ; :child_of :james_ii . """) ``SELECT`` ~~~~~~~~~~ This kind of query returns a sequence of tuples, binding variables to matching elements of a search pattern. ``SELECT`` queries are created using :meth:`prepareTupleQuery` and return results of type :class:`.TupleQueryResult`. Query result can also be serialized in a supported :class:`.TupleFormat` - in previous examples we used ``output=True`` and relied on the default ``TupleFormat.TABLE``. Here's a sample query which locates all rulers whose grandchildren inherited the crown: .. testcode:: example13 conn.setNamespace('', 'ex://') query = conn.prepareTupleQuery(query=""" SELECT DISTINCT ?name WHERE { ?grandchild :child_of/:child_of ?name . } ORDER BY ?name """) with query.evaluate() as result: for bindings in result: print(bindings.getValue('name')) Two names are returned: .. testoutput:: example13 We can also serialize the output instead of processing the result object. This time let us reverse the query and ask for rulers whose grandparents are also in the dataset: .. testcode:: example13 from franz.openrdf.rio.tupleformat import TupleFormat query = conn.prepareTupleQuery(query=""" SELECT DISTINCT ?name WHERE { ?name :child_of/:child_of ?grandparent . } ORDER BY ?name """) query.evaluate(output=True, output_format=TupleFormat.CSV) We get four results, serialized as CSV: .. testoutput:: example13 name "ex://anne" "ex://charles_ii" "ex://james_ii" "ex://mary_ii" ``ASK`` ~~~~~~~ The ``ASK`` query returns a Boolean, depending on whether the triple pattern matched any triples. Queries of this type are created using :meth:`prepareBooleanQuery`. Let's check if there were any co-regencies in the time period described by our dataset: .. testcode:: example13 query = conn.prepareBooleanQuery(query=""" ASK { ?ruler1 :reigned_from ?r1from ; :reigned_to ?r1to . ?ruler2 :reigned_from ?r2from ; :reigned_to ?r2to . FILTER (?ruler1 != ?ruler2 && ?r1from >= ?r2from && ?r1from < ?r2to) }""") print(query.evaluate()) There was one (William and Mary): .. testoutput:: example13 True ``CONSTRUCT`` ~~~~~~~~~~~~~ The ``CONSTRUCT`` query creates triples by substantiating provided templates with values resulting from matching a pattern. Queries of this kind are created using :meth:`prepareGraphQuery` and return a :class:`.RepositoryResult` - which is an iterator over the constructed triples. .. note:: Executing a ``CONSTRUCT`` query will *not* add any triples to the store. To insert the data we have to iterate over the result and add each triple using :meth:`addStatement` (or use an ``INSERT`` query). Let us consider a query that calculates a ``:sibling_of`` relationship: .. testcode:: example13 print('Size before: {0}'.format(conn.size())) query = conn.prepareGraphQuery(query=""" CONSTRUCT { ?person1 :sibling_of ?person2 . } WHERE { ?person1 :child_of ?parent . ?person2 :child_of ?parent . filter (?person1 != ?person2) . } ORDER BY ?person1""") for stmt in query.evaluate(): print('{0} <-> {1}'.format(stmt.getSubject(), stmt.getObject())) print('Size after: {0}'.format(conn.size())) The returned object is an iterator over |Statement| objects. We can also see that no data has been added to the repository. .. testoutput:: example13 Size before: 19 <-> <-> <-> <-> Size after: 19 We can also serialize the result using any of the supported :class:`RDFFormats <.RDFFormat>`: .. testcode:: example13 from franz.openrdf.rio.rdfformat import RDFFormat query.evaluate(output=True, output_format=RDFFormat.NTRIPLES) Here we use the `N-Triples`_ format. This happens to be the default, so we could have omitted the ``output_format`` argument. .. testoutput:: example13 . . . . ``DESCRIBE`` ~~~~~~~~~~~~ The ``DESCRIBE`` query returns triples that 'describe' a given set of resources. Such queries are created using :meth:`prepareGraphQuery` and return :class:`.RepositoryResult` objects. The set of resources to be processed is specified by a query pattern. The SPARQL standard does not say what triples constitute a 'description' of a particular resource. AllegroGraph will return the `Concise Bounded Description`_ of the queried resources. Let's use a ``DESCRIBE`` query to see what data do we have regarding the children of Charles I: .. testcode:: example13 query = conn.prepareGraphQuery(query=""" DESCRIBE ?child WHERE { ?child :child_of :charles_i }""") for stmt in query.evaluate(): print(stmt) In this case AllegroGraph will simply return all triples with subject in the specified set: .. testoutput:: example13 :options: +SORT (, , "1649-01-30"^^) (, , "1685-02-06"^^) (, , ) (, , "1685-02-06"^^) (, , "1688-12-11"^^) (, , ) DESCRIBE queries can be useful for exploring a dataset and learning what properties a certain object might have. The results of such queries can be serialized to any supported :class:`.RDFFormat`: .. testcode:: example13 query.evaluate(output=True, output_format=RDFFormat.NTRIPLES) .. testoutput:: example13 :options: +SORT "1649-01-30"^^ . "1685-02-06"^^ . . "1685-02-06"^^ . "1688-12-11"^^ . . .. _Concise Bounded Description: https://www.w3.org/Submission/CBD/