Arthur,
What does the spec say?
http://www.w3.org/TR/sparql11-query/#defn_aggSum
sum is defined using XSD's op:numeric-add of the evaluation of the
sum'ed expression over the grouped rows.
If one of the expressions is an error, then the whole aggregate is an
error. Unbound is not null, it's an error to try to get the value of
the variable.
You can sum over expressions where items may be unbound with either of:
SUM(IF(bound(?x),?x,0))
SUM(COALESCE(?x,0))
Andy
PS
> We have found that at least two SPARQL implementations use the SQL
> semantics
If you are naming implementations, could you name all of them?
On 14/02/14 00:29, Arthur Keen wrote:
We are developing a SPARQL 1.1 implementation, and and are hoping for
some guidance on the SPARQL 1.1. specification on how to deal with
aggregation over unbound variables.
I believe COUNT functions are the same in SQL and SPARQL. but the other
aggregates (sum/min/max/avg), seem to have different semantics (at least
per Jena).
The semantics of sum/min/max/avg are different w.r.t nulls (at least
according ot Jena).
In SQL, the sum/min/max/avg of a nullable column is the sum/min/max/avg
of the *non-null* values. For example, Suppose you have the following
data in table "foo":
name | age
----------+------
"Bob" | 5
"Bob" |
"Alice" | 3
"Alice" | 4
Then the query, "select sum(age) from foo group by name" gives this result:
name | sum
----------+-------
"Bob" | 5
"Alice" | 7
In contrast, Jena returns NULL (i.e. unbound) if there are any nulls in
the data:
-----------------------------------------------------
| name | total | cnt | cntstar | avg | min | max |
=====================================================
| "Bob" | | 1 | 2 | | | |
| "Alice" | 7 | 2 | 2 | 3.5 | 3 | 4 |
-----------------------------------------------------
Data:
@prefix foaf: <http://xmlns.com/foaf/0.1/> ..
_:a foaf:name "Alice" ..
_:a foaf:age 4 .
_:b foaf:name "Alice" .
_:b foaf:age 3 .
_:c foaf:name "Bob" .
_:c foaf:age 5 .
_:d foaf:name "Bob" ..
Query:
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name
(sum(?age) as ?total)
(count(?age) as ?cnt)
(count(*) as ?cntstar)
(avg(?age) as ?avg)
(min(?age) as ?min)
(max(?age) as ?max)
WHERE {
?x foaf:name ?name .
OPTIONAL { ?x foaf:age ?age }
}
group by ?name
We have found that at least two SPARQL implementations use the SQL
semantics for this, so it would be of benefit to the SPARQL community to
have consistent way to handle aggregation over unbound variables.
Best regards
Arthur Keen
