From: Andrew Newman <andrewfnewman@gmail.com>

Date: Wed, 26 Mar 2008 04:00:03 +1000

Message-ID: <2db5a5c40803251100p10cf0fd1w3337454e9dea67d0@mail.gmail.com>

To: "Seaborne, Andy" <andy.seaborne@hp.com>

Cc: "public-rdf-dawg-comments@w3.org" <public-rdf-dawg-comments@w3.org>

Date: Wed, 26 Mar 2008 04:00:03 +1000

Message-ID: <2db5a5c40803251100p10cf0fd1w3337454e9dea67d0@mail.gmail.com>

To: "Seaborne, Andy" <andy.seaborne@hp.com>

Cc: "public-rdf-dawg-comments@w3.org" <public-rdf-dawg-comments@w3.org>

On 25/03/2008, Seaborne, Andy <andy.seaborne@hp.com> wrote: > In [2008Mar/0013] there is a complete worked example with "UNION {}" and it shows how the results are obtained by referencing the SPARQL specification at each step. The purpose of the example in [2008Mar/0013] is so we can find the first point of difference or first point where pre-requisite knowledge [2008Mar/0029] is used. > Here are a list of my suggestions. Renaming Ù0 to Ù1 and Ù0 becomes: "Write Ù0 for the multiset consisting of no mappings, with cardinality 0. It's not expressible in SPARQL syntax. It is the UNION identity." The definition of Ù1 (currently Ù0): "Write Ù1 for the multiset consisting of exactly the empty mapping ì0, with cardinality 1. This is the join identity, it is the empty graph pattern and as a solution mapping it is represented as { {} }". If you don't like having Ù0 it doesn't matter - please consider the new definition of Ù1 (the old Ù1) separately. Maybe make it clear that in SPARQL: * X JOIN Ù1 = X and X UNION Ù0 = X but that * X UNION Ù1 != Ù1 and X JOIN Ù0 != Ù0. Another confusion with UNION in the SPARQL specification was the difference between it and the Perez paper - set union not multiset/bag union. The specification switches between multiset and set union when one definition would've done. But I think that's probably outside the scope of the changes. Other changes not related to UNION. I'd also change in 12.3: "Two solution mappings ì1 and ì2 are compatible if, for every variable v in dom(ì1) and in dom(ì2), ì1(v) = ì2(v)." Again, maybe add the stuff from the Perez paper about two disjoint domains and ì0 being compatible with everything - much less to derive - I was doing set membership instead (as that is what occurs in relational JOIN) - which seems very dumb now but it wasn't clear at the time - I haven't been able to misinterpret the Perez paper in the same way. Using "for every" is a standard but maybe "for all" makes it a bit more clear. > In [2008Mar/0015] you said of that example: > > """ > It simple to evaluate you don't need any > steps if you are UNIONing the identity for JOIN. > """ > > > I can't find mention of this - which text in the definition of the evaluation of UNION does this refer to? (For quick reference: the specification gives the definition for union of two solution multi-sets at [1]; the join identity is the multiset {{}}, cardinality 1. (sec 12.3) - I can work through that point if that would help.) > This refers to my completely made up definition which had to abide by incorrect definitions of Ù0, identities and compatibility based on previous notions about relations and misunderstanding the specification. > > If you are raising a test case against the text of the specification, could you work through the example in [2008Mar/0013] so we can identify the step or steps where you differ from the description in that message. > I think we now agree on the results of your example. Here are 5 extra tests that I'm suggesting to add. This hopefully clarifies and makes normative the specification from the test suite. Hopefully my results are understandable - I haven't used a standard syntax. Test 1 ===== This test is to show that in the empty graph pattern (5.2.1) is the JOIN identity (12.3) as mentioned at the end of 12.3.1. Data: :a :b :c . :x :y :z Query: SELECT * WHERE { { ?s ?p ?o } . {} } Results: ( [ ?s = :a, ?p = :b, ?o = :c ], [ ?s = :x, ?p = :y, ?o = :z] ) Test 2 ===== This is to show that UNION of a graph and the empty graph pattern returns all the elements in the graph plus one extra result (ì0). Data: :a :b :c . :x :y :z Query: SELECT * WHERE { { ?s ?p ?o } UNION {} } Results: ( [ ?s = :a, ?p = :b, ?o = :c ], [ ?s = :x, ?p = :y, ?o = :z], [] ) Test 3 ===== This is to show that UNION is a multiset UNION and two empty graph patterns gives two extra results (two ì0). Data: :a :b :c . :x :y :z Query: SELECT * WHERE { { ?s ?p ?o } UNION {} UNION {} } Results: ( [ ?s = :a, ?p = :b, ?o = :c ], [ ?s = :x, ?p = :y, ?o = :z], [], [] ) Test 4 ===== This is to show that compatibility in SPARQL is universal quantification (12.3 "Compatible Mappings") and that two disjoint domains results in a cross product. Data: :a :b :c . :x :y :z Query: SELECT * WHERE { { ?s ?p ?o } . { ?a ?b ?c } } Results: ( [ ?s = :a, ?p = :b, ?o = :c, ?a = :a, ?b = :b, ?c = :c ], [ ?s = :a, ?p = :b, ?o = :c, ?a = :x, ?b = :y, ?c = :z ], [ ?s = :x, ?p = :y, ?o = :z, ?a = :a, ?b = :b, ?c = :c], [ ?s = :x, ?p = :y, ?o = :z, ?a = :x, ?b = :y, ?c = :z] ) Test 5 ===== Negative syntax test. SELECT ?x WHERE { ?s ?p ?o }Received on Tuesday, 25 March 2008 18:00:36 UTC

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