- From: Orri Erling <erling@xs4all.nl>
- Date: Mon, 3 Dec 2007 23:06:29 +0100
- To: "'Adrian Walker'" <adriandwalker@gmail.com>, <jena-dev@yahoogroups.com>
- Cc: <public-rdf-dawg@w3.org>
- Message-Id: <200712032207.lB3M7PnU012642@smtp-vbr13.xs4all.nl>
Hi
Now aggregates and grouping are certainly important.
I would say that if select (count (*)) does not wwork in Virtuoso, it might as well. So in the spirit of supporting expressions in the select list we just have to rectify this. This does not merit having an extra switch in Jena.
We will have a new cut of Virtuoso Open Source Edition out in a few days most likely so the parenthesis fix can go there.
But since we are on the subject, I’d like to bring up the topic of derived tables, subqueries and quantification, since these continue to separate SPARQL from SQL even if aggregation and grouping were addressed.
It is reasonable to ask questions like what is the market share of the lowest cost retailer of gizmos in November as compared to total gizmo sales in November.
This would be a division of a sum aggregate subquery for the lowest price gizmos, i.e. where price = min (price) from November sales divided by total Nov sales of gizmos. And in order to give names to these values, should they be used in more than one expression, one would use a derived table or inline view,, as also they are called, i.e. a select inside another select’s from clause in SQL.
In a data integration situation, asking such a question in terms of SPARQL instead of SQL would even make sense, supposing multiple order processing or sales data repositories were mapped into a common ontology. Semantic is after all meant to allow using language more abstract than column names of independently designed relational schemas when talking about common things such as extended unit price. Further, while such things can be done case by
case by views and federated database or extract-transform-load and data warehousing, there is extra gain from mapping to a common CRM ontology in either case.
One might of course move the issue away from SPARQL by referring to an external inference engine that would present results of otherwise expressed calculations as triples, a bit like a SQL view presents complex queries like a table. But the problem there is that while SQL views are SQL, such rules are not SPARQL.
Existence is sort of present in SPARQL through optional and the bound check. The all quantifier does not seem very common in SQL practice. Anyway, all x reverts to not exists not x. But this cannot be said very easily in SPARQL
_____
From: public-rdf-dawg-request@w3.org [mailto:public-rdf-dawg-request@w3.org] On Behalf Of Adrian Walker
Sent: Monday, December 03, 2007 8:42 PM
To: jena-dev@yahoogroups.com
Cc: public-rdf-dawg@w3.org
Subject: Re: [jena-dev] COUNT syntax
Andy --
[apologies for cross posting -- this seems, well, important]
You wrote...
The working group is just finishing up so "official" [syntax and semantics for SPARQL aggregation] will have to wait for rechartering.
SQL made the mistake of not standardizing the semantics of aggregation, and it seems a shame that SPARQL is going down the same road. In particular, it makes it difficult to automatically generate SQL or SPARQL queries from higher level stuff.
Is there anyone in W3C who can be persuaded that this is a higher priority item?
Cheers, -- Adrian
Internet Business Logic
A Wiki and SOA Endpoint for Executable Open Vocabulary English
Online at www.reengineeringllc.com Shared use is free
Adrian Walker
Reengineering
On 12/3/07, Andreas Langegger <al@jku.at> wrote:
Hello,
it has already been noted that there are different syntax styles
around for aggregate functions like COUNT which are out of scope of
the SPARQL Rec:
https://www.blogger.com/comment.g?blogID=18002060 <https://www.blogger.com/comment.g?blogID=18002060&postID=4691605369637965896> &postID=4691605369637965896
In my case it's also an issue. The problem is, even if I just use a
"SELECT COUNT(*)" without additional parentheses - which should work
for both ARQ and Virtuoso (DBpedia e.g.) - ARQ will add the ARQ-style
parentheses when serializing the Query back into a String: "SELECT
(COUNT(*))" e.g. when executing remote queries...
I'd like to here some comments and appraisal from the community. Is
there a chance that aggregate functions become part of official
SPARQL? When? ;-)
A possible solution for ARQ would be to add a Syntax.syntaxVirtuoso,
however this is not really a solution for this issue.
Thanks,
Andy
PS@ Andy Seaborne:
Concerning my remote-bug - it was my fault. However I was already
confused enough cause I had another strange behavior with D2R
resulting in empty variable mappings when joining. Switched off the
optimizer sinze I'm using some custom Transforms... now it's working.
Thanks a lot and sorry for the false alarm!
----------------------------------------------------------
Dipl.-Ing.(FH) Andreas Langegger
Institute for Applied Knowledge Processing
Johannes Kepler University Linz
A-4040 Linz, Altenberger Strafle 69
http://www.faw.at
http://www.langegger.at
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Received on Monday, 3 December 2007 22:07:46 UTC