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 type
TupleQueryResult. Query result can also be serialized in a
supported 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:
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:
<ex://charles_i>
<ex://james_i>
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 using
prepareBooleanQuery().
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):
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 prepareGraphQuery() and return a
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 addStatement() (or use an INSERT
query).
Let us consider a query that calculates a :sibling_of
relationship:
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) .
}""")
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://james_ii> <-> <ex://charles_ii>
<ex://charles_ii> <-> <ex://james_ii>
<ex://anne> <-> <ex://mary_ii>
<ex://mary_ii> <-> <ex://anne>
Size after: 19
We can also serialize the result using any of the supported
RDFFormats:
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://james_ii> <ex://sibling_of> <ex://charles_ii> .
<ex://charles_ii> <ex://sibling_of> <ex://james_ii> .
<ex://anne> <ex://sibling_of> <ex://mary_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 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:
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> .
