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Re: [Fwd: Comments on SPARQL from the XML Query and the XSL WGs]

From: Eric Prud'hommeaux <eric@w3.org>
Date: Fri, 18 Nov 2005 06:37:52 -0500
To: "Seaborne, Andy" <andy.seaborne@hp.com>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
Message-ID: <20051118113752.GF30801@w3.org>
looks good to me. added some optional nits at lines 67, 107, 129, 164, 187

On Thu, Nov 17, 2005 at 01:10:44PM +0000, Seaborne, Andy wrote:
> 
> Proposed reply.
> 
> Notes the "in progress-ness" of double/decimal.
> 
> 	Andy
> 
> -------- Original Message --------
>   > From: Ashok Malhotra <>
>   > Date: 13 September 2005 16:28
>   >
>   > Notes on SPARQL Query Language for RDF
>   > Last Call Draft July 21, 2005
>   >
>   > The XML Query and the XSL WGs have reviewed the SPARQL last call draft.
>   > Our comments are below.  We apologize for the late delivery of these
>   > comments.  By way of explanation we mention that the WGs were on summer
>   > vacation during the month of August.
>   >
>   > 1. If RDF triples are stored in a relational database (in-memory
>   > databases are available now) then SQL can be used to query them.  SQL
>   > has much more power and is a well-known language, but there are a
>   > couple of features peculiar to RDF that are unique in SPARQL such as
>   > the OPTIONAL clause.  An analysis of how much overlap there is between
>   > SQL and SPARQL would be useful.
> 
> There are already implementations that based on SQL so useful experience
> reports are beginning to appear and will continue to appear throughout this
> phase and the CR process.
> 
> There is also work in this area appearing: for example
> "A relational algebra for SPARQL"
> Cyganiak, Richard
> http://www.hpl.hp.com/techreports/2005/HPL-2005-170.html
> 
> 
>   > 2. SPARQL cannot return a function of a stored value.  For example, if
>   > a website stores the profit of a corporation, SPARQL cannot be used to
>   > return the profit multiplied by a currency conversion factor, if a
>   > currency is specified.  This seems like a serious limitation.
> 
> In the requirements gathering process [1], the working group did not 
> identify
> this as a critical feature so it didn't make the design in this working 
> group.
> 
> There are applications already written using prototype SPARQL systems so we
> hope that what is in the current draft is sufficient for a large enough 
> class
> of applications.
> 
> [1] http://www.w3.org/TR/rdf-dawg-uc/
> 
>   > 3. The document uses a describe-by-examples style.  This is easy to
>   > understand but can give the impression that the language is more
>   > restrictive than it is.  For example, if you read section 2 you get the
>   > impression that only exact matches are supported.  Only much later do
>   > you find out that, in fact, functions and operators are also supported.
> 
> This is hard - having a document that meets the different sections of the
> SPARQL audience.  We choose the describe-by-examples style but we do 
> recognize
> that this isn't to everyone's taste.  We do provide

+                                         inline formal definitions and 

>                                                     a consolidated set of
> definitions [2] which is mechanically extracted from the document.
> 
> [2] http://www.w3.org/2001/sw/DataAccess/rq23/defns.html
> and live:
> 
> http://www.w3.org/2000/06/webdata/xslt?xslfile=http%3A%2F%2Fwww.w3.org%2F
> 2001%2Fsw%2FDataAccess%2Frq23%2Fdefns.xsl&xmlfile=http%3A%2F%2Fwww.w3.org%2F
> 2001%2Fsw%2FDataAccess%2Frq23%2F&transform=Submit
> 
> (apologies for the split link)
> 
>   > 3. How is reification supported?  That is, how does SPARQL handle
>   > values that are themselves triples?  I assume nested queries are
>   > required but there is no discussion of support for this important
>   > aspect of RDF.  Dan Connolly replied to this comment on the previous
>   > draft and explained how it could be done but an example in text would
>   > be illuminating.
> 
> An example using reification has been added (section 2.9) showing that
> reification is a vocabulary that can occur in the data.
> 
> http://www.w3.org/2001/sw/DataAccess/rq23/#queryReification
> 
> 
>   > 4. Presumably, RDF encodes metadata about data that is represented as
>   > XML.  If this is the case, why are all the XML Schema datatypes not
>   > supported?  See section 11.2.  An accompanying document
>   > [http://www.w3.org/2001/sw/BestPractices/XSCH/xsch-sw/] that discusses
>   > the use of XML Schema datatypes in RDF/OWL discusses the problems with
>   > the duration datatype and recommends the use of the two subtypes of
>   > duration xdt:yearMonthDuration and xdt:dayTimeDuration.  These
>   > datatypes are not discussed in this document.
> 
> The SPARQL draft defines the datatypes that can be expected in any SPARQL
> service.  This does not stop an implementation offering more than this 
> minimum.

In fact, considerable effort was made to make sure implementations
with extended datatypes would be compatible with those supporting the
minimal datatypes.

>   > 5. Instead of the XML Schema datatype anyURI, RDF and SPARQL uses a
>   > datatype that they refine called IRI.  The earlier version of the
>   > document said that the RDF datatype IRI is a restriction of the XML
>   > Schema datatype anyURI in that only absolute URIs are supported.  We
>   > commented that it might have been simpler just to support the XML
>   > Schema datatype and call out this restriction.  The definition of this
>   > datatype has been removed from this document and is said to be
>   > contained in the RDF Data Model.  We were unable to locate this
>   > document.
> 
> The RDF abstract syntax distinguishes between URIrefs
>   http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-URI-Vocabulary
> and typed literals
>   http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Literals
> (which use XML Schema datatypes
>   http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Datatypes-intro
> ). These can be constrasted with the following RDF/XML fragments

-                                                                s

>    <rdf:Description>
>      <foaf:homepage rdf:resource=
>         "http://www.w3.org/People/Eric/"/>
>      <foo:homepageStr rdf:datatype=
>         "http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#anyURI"
>         >http://www.w3.org/People/Eric/</foo:homepageStr>
>      <foo:shoeSize rdf:datatype=
>         "http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#integer"
>         >9</foo:shoeSize>
>      <foo:writesMailAbout rdf:resource=
>         "http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#anyURI">
>    <rdf:Description>
> 
> The RDF model distinguishes the types of the homepage and the
> homepageStr. In this example, the homepage and writesMailAbout objects
> are URIrefs (called IRIs in SPARQL). The homepageStr and shoeSize
> objects are types literals (anyURI and integer, respectively).
> 
> URIrefs identify web resources. The literals with the type anyURI
> are URIs. Thus, the strlen of the URIref
> http://www.w3.org/People/Eric/ is 10710 while the strlen of
> "http://www.w3.org/People/Eric/" is 30.
> 
> SPARQL preserves this model distinction.
> 
>   >
>   > 6. String comparison is defined only using the code point collation.
>   > Other collations are not supported.  This may be a significant
>   > limitation.
> 
> Code point collation is always required.  Access to other collections can be
> done through a custom function.

@@needs work here -- we say nothing about default vs user-supplied collations.

>   >
>   > 7. Section 3.  Decimal values cannot be written as literals.  This
>   > seems like a needless limitation.  Suggest SPARQL use the literal
>   > definitions in XPath 2.0.
> 
> The working group is investingating changing to use the literal definition 
> of
> decimal.  This requires coordination with other RDF syntax forms (N3, 
> Turtle)
> where such literals are interpreted as an XSD double.
> 
>   >
>   > 8. Section 3.1 has a very brief section entitled "Matching Arbitrary
>   > Datatypes".   There is no motivation for this and little explanation
>   > for how this works.  Motivation and detailed semantics should be 
>   provided.
> 
>   > 9. Section 11.2.1.3 introduces the function isURI.  Is this necessary?
>   > There is another function that returns the datatype of a
>   > variable.

Introducing a notation:
  <http://www.w3.org/People/Eric/> -- URIref (as described above)
  "http://www.w3.org/People/Eric/" -- URI (string, as described above)
to distinguish between 

There are typed literals, which correspond to the types distinguished
in the RDF/XML fragment above.
  STR(<http://www.w3.org/People/Eric/>) is "http://www.w3.org/People/Eric/"
  STR("http://www.w3.org/People/Eric/") is "http://www.w3.org/People/Eric/"
even if it has an xsd:datatype of xsd:anyURI.

>   > 10 sop:str returns the string representation of an r:IRI.  It's not
>   > clear what this does.  In the example, it strips the scheme from the
>   > IRI.  Does it perform any escaping or unescaping?
> 
>   > Some typos:
>   > -	Section 2: The simplest graph pattern is the triple patterns,
> 
> Fixed.
> 
>   > -	Section 11, subsection Namespaces:
>   > XML Schema datatypes with the prefix op: xs:
> 
> Fixed
> 
>   > -  Section 11.1 last para says XMLSchema[]
>   > -	There are a number of references to an XML Schema datatype called
>   > xs:int.  In fact, the XML datatype is called xs:integer.
> 
> Fixed
> 
>   >
>   > In conclusion, the XML Query and XSL working groups believe that the
>   > SPARQL specification will need considerable rework and another last
>   > call WD before it is ready to advance. In particular, we believe that
>   > the spec needs to be made more rigorous and formal, we believe that it
>   > needs to be more clearly differentiated from SQL, and it needs to be
>   > more clearly and carefully aligned with other W3C Recommendations
>   > including XML Schema and XPath 2.0.
>   >
>   > All the best, Ashok
> 
> 	Thank you very much for the comments
> 	Andy
> 
> 
> 
> 
> 

-- 
-eric

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Received on Friday, 18 November 2005 11:37:59 GMT

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