On 31/03/15 10:31, james anderson wrote:
good morning;
On 2015-03-31, at 11:04, Andy Seaborne <a...@apache.org
<mailto:a...@apache.org>> wrote:
The aggregate handle errors based on the nature of each aggregates:
e.g.
http://www.w3.org/TR/sparql11-query/#defn_aggCount
"remove error elements from N"
so count(?x) is the number of defined ?x in the group, count(*) the
number of rows, count(if(?x = 99),1,0)) the number of times 99 occurs.
Sum(?x) does not remove error or indeed non-numbers:
so sum(1+2+undef) is undef as is sum(1+"oops")
Sum(coalesce(?x,0)) on {1, 2, undef} is 3.
Sum(coalesce(?x+0,0)) on {1, 2, "oops"} is 3.
There is no canonical result for sample().
ok, let us be less specific and return to the opening question: given
the nature of sparql, which result is useful?
in addition to which, there is also a concern, how the passage in the
text which leads the section on “aggregate algebra" affects sample
argument expressions which are more complex than a single variable?
where the text specifies
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.
one reading of that text is that such expressions would certainly be
errors should they entail evaluation of an unbound marker, in which
case, the “no canonical result” interpretation for the simple variable
reference introduces a unwarranted asymmetry in the way sample
expressions are to be interpreted.
My comment "no canonical result" about SAMPLE was a general observation
that, regardless of errors, there is no defined output - it's one the
group, no guarantees as to which of the several possibilities.
What were you looking for from SAMPLE?
Andy
best regards, from berlin,
---
james anderson | ja...@dydra.com <mailto:ja...@dydra.com> | http://dydra.com
