Sparql has a notion of a "default graph" that is queried when no graph context is specified, and which (depending on the triple store) may be the union of proper graphs available in a repository, or it may be a separate, "null graph"; so far so good.
But sparql also has a keyword DEFAULT that can be specified instead of a graph name, as in
SELECT *
FROM DEFAULT
WHERE { ... }
What does this command do? I can only interpret it as an explicit way to request the same thing that would happen when there's no FROM clause at all. But is this correct? I could find no documentation about it. And what about using it in update queries, or with CLEAR, COPY, etc.? Can anyone point to documentation of the meaning and intended use of this keyword, or at least shed some light on why it exists?
When you have one or more FROM or FROM NAMED statements in a query then the dataset for the query is composed of only those graphs. Per SPARQL 1.1 Query Specification Section 13.2:
The FROM and FROM NAMED keywords allow a query to specify an RDF dataset by reference; they indicate that the dataset should include graphs that are obtained from representations of the resources identified by the given IRIs (i.e. the absolute form of the given IRI references). The dataset resulting from a number of FROM and FROM NAMED clauses is:
- a default graph consisting of the RDF merge of the graphs referred to in the FROM clauses, and
- a set of (IRI, graph) pairs, one from each FROM NAMED clause.
If there is no FROM clause, but there is one or more FROM NAMED clauses, then the dataset includes an empty graph for the default graph.
So basically the presence of those clauses creates a query dataset that potentially hides some/all graphs in the underlying dataset. Your query operates over this query dataset.
As noted in Andy's answer FROM DEFAULT is a proposed future extension to the SPARQL language that would allow explicitly referring to the datasets default graph (whatever that may be). Currently there's no standardised way to do this, so only queries that omit any FROM clauses can access the default graph unless your service provides some non-standard way to refer to it e.g. a custom URI for referencing the default graph.
For your specific example query:
SELECT *
FROM DEFAULT
WHERE { ... }
This would have the effect of forming a query dataset with a default graph using the services default and no named graphs visible i.e. any GRAPH ?g { } clauses would not match in this query