Aggregation over unbound variables

[Arthur Keen]

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