# SPARQL DESCRIBE behavior with respect to the default graph
<a name="sparql-default-describe"></a>

The SPARQL [https://www.w3.org/TR/sparql11-query/#describe](https://www.w3.org/TR/sparql11-query/#describe) query form lets you retrieve information about resources without knowing the structure of the data and without having to compose a query. How this information is assembled is left up to the SPARQL implementation. Neptune provides [several query hints](sparql-query-hints-for-describe.md) that invoke different modes and algorithms for `DESCRIBE` to use.

In Neptune's implementation, regardless of the mode, `DESCRIBE` only uses data present in the [SPARQL default graph](feature-sparql-compliance.md#sparql-default-graph). This is consistent with the way SPARQL treats datasets (see [Specifying RDF Datasets](https://www.w3.org/TR/sparql11-query/#specifyingDataset) in the SPARQL specification).

In Neptune, the default graph contains all unique triples in the union of all named graphs in the database, unless particular named graphs are specified using `FROM` and/or `FROM NAMED` clauses. All RDF data in Neptune is stored in a named graph. If a triple is inserted without a named-graph context, Neptune stores it in a named graph designated `http://aws.amazon.com/neptune/vocab/v01/DefaultNamedGraph`.

When one or more named graphs are specified using the `FROM` clause, the default graph is the union of all unique triples in those named graphs. If there is no `FROM` clause and there are one or more `FROM NAMED` clauses, then the default graph is empty.

## SPARQL `DESCRIBE` examples
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Consider the following data:

```
PREFIX ex: <https://example.com/>

GRAPH ex:g1 {
    ex:s ex:p1 "a" .
    ex:s ex:p2 "c" .
}

GRAPH ex:g2 {
    ex:s ex:p3 "b" .
    ex:s ex:p2 "c" .
}

ex:s ex:p3 "d" .
```

For this query:

```
PREFIX ex: <https://example.com/>
DESCRIBE ?s
FROM ex:g1
FROM NAMED ex:g2
WHERE {
  GRAPH ex:g2 { ?s ?p "b" . }
}
```

Neptune would return:

```
ex:s ex:p1 "a" .
ex:s ex:p2 "c" .
```

Here, the graph pattern `GRAPH ex:g2 { ?s ?p "b" }` is evaluated first, resulting in bindings for `?s`, and then the `DESCRIBE` part is evaluated over the default graph, which is now just `ex:g1`.

However, for this query:

```
PREFIX ex: <https://example.com/>
DESCRIBE ?s 
FROM NAMED ex:g1 
WHERE { 
  GRAPH ex:g1 { ?s ?p "a" . } 
}
```

Neptune would return nothing, because when a `FROM NAMED` clause is present without any `FROM` clause, the default graph is empty.

In the following query, `DESCRIBE` is used with no `FROM` or `FROM NAMED` clause present:

```
PREFIX ex: <https://example.com/>
DESCRIBE ?s 
WHERE { 
  GRAPH ex:g1 { ?s ?p "a" . } 
}
```

In this situation, the default graph is composed of all the unique triples in the union of all the named graphs in the database (formally, the RDF merge), so Neptune would return:

```
ex:s ex:p1 "a" . 
ex:s ex:p2 "c" . 
ex:s ex:p3 "b" .
ex:s ex:p3 "d" .
```