Thank you very much!
(For my future reference:
https://www.w3.org/TR/rdf-sparql-query/#emptyGroupPattern)
> Sent: Monday, June 17, 2019 at 10:55 AM
> From: "Andy Seaborne" <a...@apache.org>
> To: users@jena.apache.org
> Subject: Re: INSERT if not exists
>
> On 16/06/2019 21:01, Laura Morales wrote:
> > I would like to add a new node if and only if another node with the same
> > properties does not exists. Something like this:
> >
> >
> > INSERT {
> > ex:item-X a ex:Item;
> > ex:serial "XYZ" .
> > } if and only if there is not already another node with (ex:serial
> > "XYZ")
> >
> >
> > After trial and error I got this working
> >
> >
> > INSERT {
> > ex:item-X a ex:Item;
> > ex:serial "XYZ" .
> > }
> > WHERE {
> > FILTER NOT EXISTS {
> > [] a ex:Item;
> > ex:serial "XYZ" .
> > }
> > }
> >
> >
> > my problem is that I don't understand why it's working. First of all, is
> > the query correct?
>
> Yes.
>
> > Second, how does Jena compute this query?
>
> Try running as "SELECT * WHERE" to see that it returns either one row,
> (no variables) if the pattern does not exist or no rows when something
> exists.
>
> INSERT is a loop on the rows of the WHERE
>
> When there is one row, INSERT does soemthing.
> When there are no rows, INSERT does not happen.
>
> > Why does that work, but not this one?
> >
> > WHERE {
> > ?s ?p ?o .
> >
> > FILTER NOT EXISTS {
> > [] a ex:Item;
> > ex:serial "XYZ" .
> > }
> > }
> >
>
> Tested on an empty graph?
>
> This does not work on the empty graph because ?s ?p ?o does to match.
>
> There is an implicit empty pattern in the first update and the empty
> pattern matches (one row, no variables) even on the empty graph.
>
> Andy
>
