Hi
This is kind of a JENA question, kind of a generic SPARQL question.
The SPARQL 1.1 definition says:
----
Note that, although the result of a ListEval can be an error, and errors may be
used to group, solutions containing error values are removed at projection time.
ListEval((unbound), μ) = (error), as the evaluation of an unbound expression is
an error.
----
I have looked at this many times and cannot figure out what it means to say
that "errors may be used to group". To make sense of this requires that errors
be comparable for equality (otherwise you can't group …), which I don't see any
definition of anywhere in the SPARQL semantics.
I don't even understand how it works in a really simple query grouping
solutions from a basic graph pattern, for example:
select whatever from ..
where
{
?a ?b ?c
}
group by ?a+1, ?b+2
Suppose:
-) ?a+1 produces a dynamic error but ?b+2 does not for one choice of ?a and ?b
-) ?a+1 does not produce a dynamic error but ?b+2 does for a different choice
of ?a and ?b
Are these two choices place in the same group?
And what happens if for two choices of ?a and ?b, ?a+1 and ?b+2 both produce
produce dynamic errors, but different ones (we we have 4 different kinds of
errors)? Are they in the same group? And are they in the same group as the
previous example?
My gut says that what was intended here was SQL style null groupings (which are
horrible, but ..). But the mathematics in the definition isn't clear.
Any SPARQL gurus know what is meant to happen for grouping here?
thanks
graham
