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
