Querying multiple contexts — AllegroGraph Python client 104.2.0 documentation (original) (raw)
Example 7: Querying multiple contexts¶
The purpose of this example is to see how data imported into multiple contexts (like that from Example 6: Importing triples) behaves when queried using various methods. This exampe covers only the results of basic queries. The subject is explored in more detail in Example 10: Graphs in SPARQL.
Let us start by creating a connection:
from franz.openrdf.connect import ag_connect
conn = ag_connect("python-tutorial", create=True, clear=True)
and adding a few triples in the default context:
from franz.openrdf.query.query import QueryLanguage
conn.addData(""" ex://default1 ex://p1 1 . ex://default2 ex://p2 2 . ex://default3 ex://p3 3 .""")
We can add data to another contect by using the optional contextparameter of addData():
context = conn.createURI('ex://context') conn.addData(""" ex://context1 ex://p1 1 . ex://context2 ex://p2 2 . ex://context3 ex://p3 3 .""", context=context)
Let’s try a getStatements() call first:
p1 = conn.createURI('ex://p1') with conn.getStatements(None, p1, None, None) as result: for row in result: print(row.getSubject())
This loop prints out a mix of triples from the default context and from the named context.
SPARQL queries behave in a different way. When a graph clause is present, as in the following code, triples that are not in a named context will not be examined:
query_string = """ SELECT DISTINCT ?s WHERE { graph ?g { ?s ?p ?o filter(?o > 2). }} order by ?s""" tuple_query = conn.prepareTupleQuery( QueryLanguage.SPARQL, query_string) with tuple_query.evaluate() as result: for bindings in result: print(bindings[0])
Only the context3 triple is printed:
What happens if we issue a trivial query without mentioning graph?
query_string = """ SELECT DISTINCT ?s WHERE { ?s ?p ?o . } order by ?s""" tuple_query = conn.prepareTupleQuery( QueryLanguage.SPARQL, query_string) with tuple_query.evaluate() as result: for bindings in result: print(bindings[0])
This prints all triples, just like a getStatements() call.
ex://context1 ex://context2 ex://context3 ex://default1 ex://default2 ex://default3
But this behavior can be altered by setting a query option. AllegroGraph allows such options to be set by defining a prefix.
query_string = """ PREFIX franzOption_defaultDatasetBehavior: franz:rdf SELECT DISTINCT ?s WHERE { ?s ?p ?o . } order by ?s""" tuple_query = conn.prepareTupleQuery( QueryLanguage.SPARQL, query_string) with tuple_query.evaluate() as result: for bindings in result: print(bindings[0])
Now only the default context is matched by simple pattern (i.e. ones not wrapped in graph ?g { ... })