A SPARQL dataset refers to the dataset notion from the SPARQL specification, within dotNetRDF it may refer to the concrete ISparqlDataset interface which represents datasets for in-memory queries or it may refer to the dataset description associated with a query/update.
The dataset for a SPARQL Query or Update consists of a default graph and zero or more named graphs, the default graph is often also referred to as the unnamed graph.
SPARQL does not mandate what the default graph is so this can and often is dataset specific, see the later sections of this page for how to construct different types of dataset for use with the libraries in-memory SPARLQ engine.
The dataset description of a query consists of the
FROM NAMED clauses present in the query, these indicate to a query engine which graph(s) to use and where to use them when answering queries.
Graphs specified in the
FROM clause are used to form the default graph, this is the graph that queries operate over except when a
GRAPH clause is encountered.
Graphs specified in the
FROM NAMED clauses are named graphs that may be accessed using a
GRAPH clause in your query.
INSERT/DELETE command in SPARQL Update allows the dataset to be specified in several ways:
WITHclause specifies a default graph for the
DELETEportions of the query
USINGclauses specifies the graphs that form the default graph for the
WHEREportion of the update. If there are no
WITHclause specifies the default graph, if there is no
WITHthen the default graph is dataset specific.
USING NAMEDclause specifies the named graphs for the
WHEREportion of the update.
The ISparqlDataset interface is used to represent a dataset over which queries and updates are applied. dotNetRDF includes a number of implementations of this primarily for wrapping a
IInMemoryQueryableStore i.e. an in-memory
ITripleStore so that it can be queried.
How you construct your dataset can affect the results of your query due to SPARQL dataset descriptions as already discussed on this page. For our examples here we will use the InMemoryDataset which may be constructed in a number of ways.
If your queries involve a lot of named graphs then you may get better performance by using an InMemoryQuadDataset instead. This has identical constructors to the ones shown in the following examples.
The default behavior of dotNetRDF is to treat the unnamed graph as the default graph and all other graphs as named graphs. You get this type of dataset when constructing like so:
//Construct a fresh dataset ISparqlDataset ds = new InMemoryDataset();
Or if you construct from an existing triple store:
TripleStore store = new TripleStore(); //Assume it gets filled with data from somewhere... //Construct a dataset ISparqlDataset ds = new InMemoryDataset(store);
Specific Default Graph
If you have a specific graph that you wish to have treated as the default graph you can create a dataset like so:
TripleStore store = new TripleStore(); //Assume it gets filled with data from somewhere //Construct a dataset using a specific graph as the default ISparqlDataset ds = new InMemoryDataset(store, new Uri("http://example.org/default-graph"));
Union Default Graph
Union default graph is a special behavior whereby the default graphs acts as if it were the union of all the graphs in the store, it may be constructed like so:
TripleStore store = new TripleStore(); //Assume it gets filled with data from somewhere //Construct a dataset using a union default graph ISparqlDataset ds = new InMemoryDataset(store, true);