Export graph

[Laurent Pierre]

Hi,

I try exporting a graph and I get this message :

The null graph contains triples that will not be exported. You can force the export, to ignore this issue

Any idea for fixing that ?

Laurent

[Armando Stellato]

Dear Laurent,

 

this means that, for some reason, some triples are stored in the null graph, that is the only “unnamed” graph in the list of graphs of the repository.

 

VB never writes in it, there are however ways in which this can be written, e.g.

 

1. A SPARQL UPDATE with no specification of the destination graph. In that case, the default write graph is the null graph
2. Loading data expressed as quadruples (i.e. using some quadruple format, such as .NQUADS). In that case, if I’m not wrong, the explicit graphs expressed in the data should prevail over the single graph specified in the import

 

To understand the reason for your case first, did you incur in any of the above cases?

 

Going to a solution, one possibility is to use a SPARQL UPDATE, e.g.

 

DELETE { GRAPH rdf4j:nil { ?s ?p ?o } } WHERE { ?s ?p ?o }

 

With prefix for rdf4j being:

http://rdf4j.org/schema/rdf4j#

 

Pls notice that SPARQL does not have a way to address the null graph, so this http://rdf4j.org/schema/rdf4j#nil is a specific SPARQL language extension existing in RDF4J

 

Kind Regards,

 

Armando

 

 

 

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[Laurent Pierre]

Thank you Armando,

As a by-pass I export data from GraphDB for back up.

Kind regards,

Laurent

[Armando Stellato]

Hi Laurent,

 

sure you can do it. However, let me add something, because that is not really solving your issue:

 

1. In VB you can do the export by forcing it, the message just says that the data in the null graph will be ignored
2. I would in any case check what is inside your null graph, because from there, it won’t be manageable

 

Kind Regards,

 

Armando

 

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[Laurent Pierre]

Dear Armando,

The problem is when I run the query to explore what's in the null graph :

PREFIX rdf4j: <http://rdf4j.org/schema/rdf4j#>

SELECT *
WHERE {
GRAPH rdf4j:nil
    {
    ?s ?p ?o .
    }
} LIMIT 10

I get nothing;

KInd regards,

Laurent

[Armando Stellato]

Oh, thanks, that smells of a bug then! (unless rdf4j:nil is not working properly).

We’ll investigate into it, thanks for reporting :-)

 

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[Roland Wingerter]

Hi Laurent,

you can also use GraphDB Workbench to inspect the contents of the "null graph". Go to Explore > Graphs overview, then click on "The default graph". 

Kind regards

Roland