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Re: Further comments on SPARQL 1.1

From: Steve Harris <steve.harris@garlik.com>
Date: Fri, 26 Feb 2010 16:36:19 +0000
Cc: <public-rdf-dawg-comments@w3.org>
Message-Id: <E1ED75DB-6CDB-450C-9CB5-39D0A4F55819@garlik.com>
To: Rob Vesse <rav08r@ecs.soton.ac.uk>
Responding as an individual, not representing the group...

On 25 Feb 2010, at 15:24, Rob Vesse wrote:

> With regards to my previous comments and the Working Group’s  
> response [1] I would like to make some further comments.
> Aggregates – I do not like the proposed SEPARATOR syntax for  
> GROUP_CONCAT at all, if you are going to include GROUP_CONCAT simply  
> leave it as a single argument with a standard separator and then  
> specify that fn:string-join() must be supported by SPARQL 1.1 as a  
> function since I believe the WG intends to specify a subset of XPath  
> and other common functions that implementations should support?

Referring to XPath functions directly is not helpful, as they're  
orthogonal to the semantics of aggregates. Functions take a list of  
values, perform some internal operation on them, and return a value.  
Aggregations take a list (actually a multi-set I believe) of  
expressions, evaluate them with respect to a binding set, apply some  
set function, and then return a value. These are not compatible  

If you have something like:

<x> <p> "1" .
<x> <p> "2" .

    <x> <p> ?x .

you will get back something like:

?c = "1 | 2 |"

But that's because it evaluates to

Aggregation((), {?x, "|"}, GROUP_CONCAT, [BS]) as per §9.2 of SPARQL  
1.1 draft.

Which evaluates to GROUP_CONCAT({"1", "|", "2", "|"})

The SEPARATOR syntax comes from MySQL, which is where GROUP_CONCAT  
originates from, it's necessary in SQL for the same reason it is in  

I'm not planning to add order ORDER BY aggregate syntax in SPARQL 1.1,  
but I'm leaving it open for future WGs.

Hope this helps,

Steve Harris, Garlik Limited
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+44 20 8973 2465  http://www.garlik.com/
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Registered office: Thames House, Portsmouth Road, Esher, Surrey, KT10  
Received on Friday, 26 February 2010 16:36:54 UTC

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