SPARQL query forms — AllegroGraph Python client 104.2.0 documentation (original) (raw)
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:
from franz.openrdf.connect import ag_connect
conn = ag_connect("python-tutorial", create=True, clear=True)
We’ll need some sample data to illustrate all the query types. Our dataset will contain information about rulers of 17th century England.
conn.addData(""" @prefix : ex:// . @prefix xsd: http://www.w3.org/2001/XMLSchema# .
: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 prepareTupleQuery() and return results of typeTupleQueryResult. Query result can also be serialized in a supported TupleFormat - in previous examples we usedoutput=True and relied on the default TupleFormat.TABLE.
Here’s a sample query which locates all rulers whose grandchildren inherited the crown:
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:
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:
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:
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 usingprepareBooleanQuery().
Let’s check if there were any co-regencies in the time period described by our dataset:
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):
CONSTRUCT¶
The CONSTRUCT query creates triples by substantiating provided templates with values resulting from matching a pattern. Queries of this kind are created using prepareGraphQuery() and return aRepositoryResult - 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 addStatement() (or use an INSERTquery).
Let us consider a query that calculates a :sibling_ofrelationship:
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.
Size before: 19 ex://anne <-> ex://mary_ii ex://charles_ii <-> ex://james_ii ex://james_ii <-> ex://charles_ii ex://mary_ii <-> ex://anne Size after: 19
We can also serialize the result using any of the supportedRDFFormats:
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.
ex://anne ex://sibling_of ex://mary_ii . ex://charles_ii ex://sibling_of ex://james_ii . ex://james_ii ex://sibling_of ex://charles_ii . ex://mary_ii ex://sibling_of ex://anne .
DESCRIBE¶
The DESCRIBE query returns triples that ‘describe’ a given set of resources. Such queries are created using prepareGraphQuery()and return 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 theConcise 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:
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:
(ex://charles_ii, ex://reigned_from, "1649-01-30"^^http://www.w3.org/2001/XMLSchema#date) (ex://charles_ii, ex://reigned_to, "1685-02-06"^^http://www.w3.org/2001/XMLSchema#date) (ex://charles_ii, ex://child_of, ex://charles_i) (ex://james_ii, ex://reigned_from, "1685-02-06"^^http://www.w3.org/2001/XMLSchema#date) (ex://james_ii, ex://reigned_to, "1688-12-11"^^http://www.w3.org/2001/XMLSchema#date) (ex://james_ii, ex://child_of, ex://charles_i)
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 RDFFormat:
query.evaluate(output=True, output_format=RDFFormat.NTRIPLES)
ex://charles_ii ex://reigned_from "1649-01-30"^^http://www.w3.org/2001/XMLSchema#date . ex://charles_ii ex://reigned_to "1685-02-06"^^http://www.w3.org/2001/XMLSchema#date . ex://charles_ii ex://child_of ex://charles_i . ex://james_ii ex://reigned_from "1685-02-06"^^http://www.w3.org/2001/XMLSchema#date . ex://james_ii ex://reigned_to "1688-12-11"^^http://www.w3.org/2001/XMLSchema#date . ex://james_ii ex://child_of ex://charles_i .