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Re: Semantics of SUM

From: Andy Seaborne <andy.seaborne@talis.com>
Date: Wed, 11 Nov 2009 10:46:45 +0000
Message-ID: <4AFA9615.8060202@talis.com>
To: Steve Harris <steve.harris@garlik.com>
CC: RDF Data Access Working Group <public-rdf-dawg@w3.org>


On 10/11/2009 18:47, Steve Harris wrote:
> On 10 Nov 2009, at 18:25, Andy Seaborne wrote:
>> On 10/11/2009 17:57, Steve Harris wrote:
>>>> > Consensus that SUM/AVG should use same semantics as +
>>>>
>>>> Clarification: errors not in a group means that what would be
>>>>
>>>> 1 + error + 2 => 3
>>>>
>>>> which is not the same as +
>>>
>>> Yep, which I think is why they are in the group, and why COALESCE is
>>> important.
>>
>> I don't see that COALESCE helps because there isn't an intermediate
>> variable representing the sum so far.
>
> I think the idea was something like:
>
> SELECT SUM(COALESCE(xsd:double(?x), 0))
> WHERE {
> <x> <value> ?x .
> }

Thanks - I'd missed that.

However, it does not quite follow the semantics of "+" because it 
changes the datatype:

SUM(?x | ?x{1,2,3} ) ==> "6"^^xsd:integer

but

SUM(COALESCE(xsd:double(?x), 0) | ?x{1,2,3} ) => "6"^^xsd:double

Using xsd:integer does not work e.g. 1, 1.5, 3 !=> "5.5"^^xsd:decimal



I don't see how to write a MIN that works using COALESCE because there 
is no natural value that plays the role zero did (e.g. MIN over strings 
mixed with non-strings).

MIN(IF(datatype(?x)=xsd:String, ?x, ???))

And xsd:integers are arbitrary length so the COALESCE idiom is hard for 
MIN (either definition).


If SUM follows +, does MIN follow < or ORDER BY?
http://lists.w3.org/Archives/Public/public-rdf-dawg/2009AprJun/0250.html


	Andy

>
> That parallels what you do in SQL:
>
>  > SELECT * FROM C;
> +------+
> | val |
> +------+
> | 1 |
> | 2 |
> | 5 |
> | 23 |
> | NULL |
> +------+
>
>  > SELECT SUM(COALESCE(val, 100)) FROM C;
> +-------------------------+
> | SUM(COALESCE(val, 100)) |
> +-------------------------+
> | 131 |
> +-------------------------+
>
> - Steve
>
Received on Wednesday, 11 November 2009 10:47:06 GMT

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