Advanced Querying

The Getting Started Tutorial covered the basic of querying with kDB. This tutorial covers advanced topics, such as the use of bindings and of the RDFEnvironment.

Bindings

Bindings allow to reference values used in queries by a keyword which is then substitude by the value during the query execution. Lets consider the following SQL query, which does not use bindings:

SELECT * FROM triples_stores WHERE name='http://askco.re/graphs#default';

This query can be rewritten using the binding :name as:

SELECT * FROM triples_stores WHERE name=:name;

And then before executing the query, it is necesserary to specify the value for :name.

The same principle can be applied to SPARQL query, execpt the name of the binding must start with %, for instance:

SELECT ?y ?z WHERE { %subject ?y ?z . };

Bindings API

Bindings are added to the query using the addBinding functions of the knowDBC::Query, as demonstrated bellow:

#include <knowDBC/Query.h>
#include <knowDBC/Result.h>

// Assuming a 'connection' was created.

// The following creates an SQL query, listing the content of the triples_stores table.
knowDBC::Query query = connection.createSQLQuery("SELECT * FROM triples_stores WHERE name=:name");

// Bind the value "http://askco.re/graphs#default" to the binding ":name", note that ':'
// is required.
query.bindValue(":name", "http://askco.re/graphs#default");

// The following execute either the SQL or SPARQL query
knowDBC::Result result = query.execute();

# Assuming a 'connection' was created.

# The following creates an SQL query, listing the content of the triples_stores table.
query = connection.createSQLQuery("SELECT * FROM triples_stores WHERE name=:name")

# Bind the value "http://askco.re/graphs#default" to the binding ":name", note that ':'
# is required.
query.bindValue(":name", "http://askco.re/graphs#default")

# Assuming a 'connection' was created.

# The following creates an SQL query, listing the content of the triples_stores table.
query = connection.createSQLQuery "SELECT * FROM triples_stores WHERE name=:name"

# Bind the value "http://askco.re/graphs#default" to the binding ":name", note that ':'
# is required.
query.bindValue ":name", "http://askco.re/graphs#default"

RDF Environment

The RDF Environment is used to define the context in which a SPARQL query is executed. It contains the following information:

base url the base URL used for all relative links in the query (can be overwritten with the @base keyword)
default dataset the main dataset used for querying (can be overwritten with the FROM keyword)
named datasets a list of datasets that can be referenced with their names in SPARQL query (can be overwritten with the FROM NAMED keywords).
services a list of RDF Services that can be used in the query (usually for executing remote queries).

#include <knowDBC/Query.h>
#include <knowDBC/Result.h>
#include <kDB/Repository/RDFEnvironment.h>

// Assuming a 'connection' was created.

// The following creates an SPARQL Query and set the base, default dataset and add a named dataset
knowDBC::Query query = connection.createSPARQLQuery(
  kDB::Repository::RDFEnvironment()
    .setBase("file:///home/kdb/query.rq#")
    .setDefaultDataset(connection.graphsManager()->getTripleStore("http://askco.re/graphs#default"_kCu))
    .addNamedDataset(connection.graphsManager()->getTripleStore("http://askco.re/graphs#info"_kCu))
    ...,
  "...");

from knowCore import Uri

# Assuming a 'connection' was created.

# The following creates an SPARQL Query and set the base, default dataset and add a named dataset
query = connection.createSPARQLQuery(
  kDB.Repository.RDFEnvironment()
    .setBase(Uri("file:///home/kdb/query.rq#"))
    .setDefaultDataset(connection.graphsManager().getTriplesStore(Uri("http://askco.re/graphs#default")))
    .addNamedDataset(connection.graphsManager().getTriplesStore(Uri("http://askco.re/graphs#info")))
    ...,
  "...");

# Assuming a 'connection' was created.

# The following creates an SPARQL Query and set the base, default dataset and add a named dataset
query = connection.createSPARQLQuery(
  kDB.Repository.RDFEnvironment()
    .setBase(kCu("file:///home/kdb/query.rq#"))
    .setDefaultDataset(connection.graphsManager().getTriplesStore(kCu("http://askco.re/graphs#default")))
    .addNamedDataset(connection.graphsManager().getTriplesStore(kCu("http://askco.re/graphs#info")))
    ...,
  "...");

RDF Union --------- It is also possible to query against the union of two RDF Datasets.

#include <knowDBC/Query.h>
#include <knowDBC/Result.h>
#include <knowDBC/Result.h>
#include <kDB/Repository/RDFEnvironment.h>
#include <kDB/Repository/DatasetsUnion.h>

// Assuming a 'connection' was created.

// The following creates a dataset union
kDB::Repository::DatasetsUnion dsu;
dsu.add(connection.graphsManager()->getTripleStore("http://askco.re/graphs#default"_kCu));
dsu.add(connection.graphsManager()->getTripleStore("http://askco.re/graphs#info"_kCu));

// The following creates an SPARQL Query against that union
knowDBC::Query query = connection.createSPARQLQuery(
  kDB::Repository::RDFEnvironment(dsu),
  "...");

from knowCore import Uri

# Assuming a 'connection' was created.

# The following creates a dataset union
dsu = kDB.Repository.DatasetsUnion()
dsu.add(connection.graphsManager().getTriplesStore(Uri("http://askco.re/graphs#default")))
dsu.add(connection.graphsManager().getTriplesStore(Uri("http://askco.re/graphs#info")))

# The following creates an SPARQL Query against that union
query = connection.createSPARQLQuery(
  kDB.Repository.RDFEnvironment(dsu),
  "...");

# Assuming a 'connection' was created.

# The following creates a dataset union
dsu = KDB::Repository::DatasetsUnion.new
dsu.add connection.graphsManager.getTriplesStore kCu("http://askco.re/graphs#default")
dsu.add connection.graphsManager.getTriplesStore kCu("http://askco.re/graphs#info")

# The following creates an SPARQL Query against that union
query = connection.createSPARQLQuery
  kDB.Repository.RDFEnvironment.new(dsu),
  "..."