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Re: Semantics of SUM

From: Steve Harris <steve.harris@garlik.com>
Date: Thu, 12 Nov 2009 22:54:51 +0000
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
Message-Id: <3B5E6819-BB1C-482B-9479-4A2EDAE7127E@garlik.com>
To: Andy Seaborne <andy.seaborne@talis.com>
On 12 Nov 2009, at 20:10, Andy Seaborne wrote:
>>> However, it does not quite follow the semantics of "+" because it
>>> changes the datatype:
>>>
>>> SUM(?x | ?x{1,2,3} ) ==> "6"^^xsd:integer
>
> Notation was meant to be SUM(?x) over a group with ?x=1 ,?x=2, ?x=3
> It was a bit terse - had been writing comprehensions just before  
> hand so it was obvious to me, at the time.

Hm, yeah, I guessed that, but the rest I didn't follow. Makes sense  
now though.

>>> but
>>>
>>> SUM(COALESCE(xsd:double(?x), 0) | ?x{1,2,3} ) => "6"^^xsd:double
>
> Using
>   SUM(COALESCE(xsd:double(?x), 0))
> for group members ?x=1,2,3 (xsd:integers) results in xsd:double, not  
> xsd:integer as it would for 1+2+3

No, sure, but COALESCE(?x, 0) would in this case.

>>> Using xsd:integer does not work e.g. 1, 1.5, 3 !=>  
>>> "5.5"^^xsd:decimal
>
> Encoding the full XSD hierarchy to minimise the promotion as + used  
> natively does, and cope with errors/non-numbers might be:
>
> COALESCE(xsd:integer(?x), xsd:decimal(?x), xsd:double(?x), 0)
>
> which I don't think is a serious contender on practical grounds.

No, but I don't imagine there are too many situations where users  
would want the exact semantics, do you have good examples?

> I think ending up with xsd:double, not xsd:integer, in the potential  
> present of errors, is significant so maybe a way to indicate that  
> errors be excluded might be a better.  However, a plethora of  
> options is also bad design.  Hmm.

I think you could do that with COALESCE(?x), if none of the values  
passed to COALESCE are not type errors or bound, you get an unbound  
value returned, so it will be dropped. I think.

- Steve

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Received on Thursday, 12 November 2009 22:55:26 GMT

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