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Re: SPARQL 1.1 Query Aggregates Review

From: Steve Harris <steve.harris@garlik.com>
Date: Mon, 28 Feb 2011 23:04:46 +0000
Cc: SPARQL Working Group <public-rdf-dawg@w3.org>
Message-Id: <61322E50-8C5C-4592-BC0D-07CB9DECDAFF@garlik.com>
To: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Thanks Birte, that's very helpful.

I've just committed version 1.212 which addresses these problems.

I've also added an example showing how error are handled, and fixed some more typographical errors.

- Steve

On 2011-02-25, at 22:06, Birte Glimm wrote:

> Steve, others, 
> I have now reviewed Section 11 (Aggregates) and I attach my comments below. Mostly there are some typos and smaller corrections, but I can't make sense out of 11.6. I gues at least an example is need and probably a bit of text to put that subsection into context. 
> Best regards,
> Birte
> 11 Aggregates
> ...
> Aggregates are used where the querier wishes to see a result which is
> compute*d* (not computer)
> 11.1
> ...
> In aggregate queries and sub-queries variables that appear in the query
> pattern, but are not grouped by ...
> "not grouped by" sounds a bit strange to me, I would prefer just "but
> are not grouped" or "but not in the GROUP BY clause"
> "It should be noted that <link>as per functions</link>, aggregate expressions must
> be alised inorder to project them from queries or subqueries. In the
> example obove this is done using the variable ?totalPrice. It is an
> error for aggregates to project variables with a name already used in
> other aggregate projections."
> For me the link does not work. 
> Should "alised" be aliased?
> obove <- above
> 11.2
> ...
> "Within GROUP BY clases the assignment keyword, AS, may be used. Such
> as GROUP BY (?x + ?y AS ?z). "
> clases <- clauses
> Such as... Is not really a complete sentence and rather belongs to the previous
> sentence. 
> ...
> "We can then Apply the set function Avg() to the group solutions, using the Aggregation() algebra function, as Aggregation((?y), Avg, {}, G), giving:..."
> apply (lowercase)
> I wondered what the empty set in the Aggregation() algebra function
> was, but in order to get any clues, one has to click on the link and
> read the algebra section. Maybe one can shortly comment on that
> already informally in Section 11.2 or choose an example that also uses
> that feature?
> 11.4 
> ...
> "Note that it would not be legal to project STR(?z) as this is not a
> simple variable expression."
> I guess it would be legal to select ?newZ if I were to use (STR(?z) AS
> ?newZ) right? If so, it would be nice to point that out too. 
> "Other expresisons, not using GROUP BY variables, or aggregates may have non-deterministic values projected from their groups using the SAMPLE() aggeregate."
> aggeregate <- aggregate
> 11.5
> "The set functions which underlie SPARQL aggregates all have a common
> signature: SetFunc(M, err), or SetFunc(M, err, scalarvals, ...) where M is a
> multiset of lists, err is a value indicating whether the evaluation of any of the
> expressions evaluated with respect to Ω "
> Here we have scalarvals again, so it might really be helpful to say a
> bit about that in the example in 11.2. Also I find Sigma a bit out of
> ontext here, it's the first time used in this section. Could the text
> not just say "a solution sequence"?
> "The name is retained due to the comonality with SQL Set Functions,
> which also operate over multisets."
> commonality
> "... Systems may choose to expand this set with *local using extensions*, using the same notation as for functions and casts. "
> local extensions?
> 11.6
> "In order to project values from (sub-)queries using aggregate values,
> a Solution Multiset is constructed where each solution comprises the
> results of the Aggregate functions which share a key."
> Why upper case Solution Multiset and Aggregate functions?
> I can't really make sense out of that section. Maybe an example would
> help, but I neither see in what context I would need such aggregate
> joins nor how a query would look like that uses/requires this feature.
> -- 
> Dr. Birte Glimm, Room 309
> Computing Laboratory
> Parks Road
> Oxford
> OX1 3QD
> United Kingdom
> +44 (0)1865 283520

Steve Harris, CTO, Garlik Limited
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Received on Monday, 28 February 2011 23:05:24 UTC

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