sparql - Jena ARQ returns non-empty result for AVG() if there is no matching triple pattern -

i have query uses avg() operator:

select (avg(?z) ?avg) { ?x <http://ex.com/value> ?z } 

let's imagine triple store doesn't have triples matching given triple pattern, expect (at least do) query should return empty result. , virtuoso returns empty result, can use dbpedia's sparql endpoint check (execute).

but fuseki , jena arq return non-empty result: 0. can check on sparql.org (execute).

is possible configure jena arq return empty result given query? if so, how?

we expect (at least do) query should return empty result. , virtuoso returns empty result, can use dbpedia's sparql endpoint check.

but fuseki , jena arq return non-empty result: 0. can check on sparql.org.

i don't know whether can change behavior of avg, shouldn't, because jena doing right thing here, , virtuoso (the endpoint dbpedia) doing wrong. sparql 1.1 standard defines avg return when group empty:

18.5.1.4 avg

the avg set function calculates average value expression on group. defined in terms of sum , count.

definition: avg numeric avg(multiset m)

avg(m) = "0"^^xsd:integer, where count(m) = 0

avg(m) = sum(m) / count(m), count(m) > 0

that said, can use if check whether count zero, , return undefined value in case, , average otherwise. should work endpoint, too, not arq. used values introduce variable undefined value here, use expression produce error, e.g., 1/0. (of course, danger there implementations can extend behavior of operators, it's pretty hard guarantee particular expression error.)

select (if(count(?x) = 0,?undef,avg(?x)) ?average) {       values ?x { 2 3 4 }       values ?undef { undef } } group ?undef 

----------- | average | =========== | 3.0     | ----------- 

and in case there no values ?x:

select (if(count(?x) = 0,?undef,avg(?x)) ?average) {   values ?x {}   values ?undef { undef } } group ?undef 

----------- | average | =========== |         | -----------