INSERT if not exists

[Laura Morales]

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? Second, how does Jena compute this query? Why does that work, 
but not this one?

    WHERE {
        ?s ?p ?o .

        FILTER NOT EXISTS {
            [] a ex:Item;
               ex:serial "XYZ" .
        }
    }