- From: Hugh Glaser <hugh@glasers.org>
- Date: Sun, 25 Aug 2019 11:02:11 +0100
- To: Henry Story <henry.story@bblfish.net>
- Cc: Daniel Hernandez <daniel@degu.cl>, "semantic-web@w3.org" <semantic-web@w3.org>
The blank node comments here got me pondering. Sorry, I'm a bit of a newbie with blank nodes (I never use them, and perhaps this posting indicates why), so this may all be well-known and in the docs. But can I check my understanding here, please. As soon as you start to query a graph with blank nodes in practice, I think things gets pretty weird, and even unpredictable. Yes, in the RDF model, I'm sure it is all clear. But serialising changes everything. I guess the simplest thing I am thinking of is something like: id:id1 ns:p1 [ ns:p2 id:id2 ] . With these sorts of queries: Q1) ?s ns:p1 ?o . Q2) id:id1 ?p ?o . Q3) ?s ?p id:id2 . Depending on doing SELECT v. CONSTRUCT (or with a Linked Data wrapper around the store, accessing id:id1 and id:id2), and what sort of content I might be negotiating, I will get various sorts of results. They will usually have something like _:genid1 or <ns:p1 rdf:nodeID="genid1"/> or whatever, and it may be in multiple documents. But I *think* that what happens is that the connection between id:id1 and id:id2 is kept (Q1) or lost (Q2 + Q3), entirely dependent on how you choose to get the RDF out of your store. I hope I haven't misunderstood things - I have tried it on some stores, but that isn't a reliable way of working out what is meant to happen by definition of SPARQL etc.. Have I got it wrong? And is that also really what SQL does, as you say? [Exercise for the reader: create a Category Theory description of what blank nodes actually do in the whole Semantic Web ecosystem :-) ] Best Hugh > On 25 Aug 2019, at 06:19, Henry Story <henry.story@bblfish.net> wrote: > > Continuing this thread that started with the funny story on the NULL > vanity licence plate reported here: > https://mashable.com/article/dmv-vanity-license-plate-def-con-backfire/ > > I just came across work by Ryan Wisnesky on Algebraic Databases, where > the authors formalizes DBs in terms of Category Theory, in order to build provably > correct ways to transform data. > > In that formalization, for which they have software tools, they give an clear > explanation of NULLs in SQL databases that make each > NULL different. In the talk linked to below Ryan Wisnesky actually gives them > different subscripts. > > In that way nulls in DBs are very different from nulls in > Java - which can be compared for equality and for which there exists only one > instance - and very similar to blank nodes on the semantic web. > > See the presentation ”Algebraic Databases” on his web site > https://www.wisnesky.net/ > Or other content I found on this work > https://twitter.com/bblfish/status/1165195822625153024 > > Henry Story > > >> On 13 Aug 2019, at 15:53, Daniel Hernandez <daniel@degu.cl> wrote: >> >> SQL nulls are similar in some aspects to Codd nulls. A difference is that SQL nulls do no provide guaranty that the value exists. Blank nodes, on the other hand, are similar to marked nulls. We study the application to SPARQL of SQL techniques to approximate certain answers in: "Certain Answers for SPARQL with Blank Nodes." However, we founded a unique dataset using blank nodes as unknown values (Wikidata). I am curious if you know another. >> >> On Tue, Aug 13, 2019 at 3:53 AM, Franconi Enrico <franconi@inf.unibz.it> wrote: >>> The situation is slightly more complex than that. >>> NULL values in standard SQL are exactly defined as letting any equality involving a NULL value fail. >>> Note that the string 'NULL' represents a NULL value, namely if you type the string NULL into a cell of type STRING then it is understood to be a NULL value. >>> This is where the implementors failed: a NULL value is never equal to itself. >>> This can be understood with the following standard SQL example (try it!). >>> >>> With the database: >>> >>> TABLE: col1 | col2 >>> -----+----- >>> a | b >>> b | NULL >>> >>> the query (meant to be the identity query, namely returning the input table itself): >>> >>> SELECT * FROM TABLE >>> WHERE TABLE.col1 = TABLE.col1 AND TABLE.col2 = TABLE.col2 ; >>> >>> gives the result: >>> >>> col1 | col2 >>> -----+----- >>> a | b >>> >>> In SQL, the query above returns the table TABLE if and only if the table TABLE does not have any NULL value, otherwise it returns just the tuples not containing a NULL value, i.e., in this case only the first tuple <a,b>. Informally this is due to the fact that a SQL NULL value is never equal (or not equal) to anything, including itself. This is because a SQL NULL value represents the absence of a value. >>> >>> Note that this is where SQL NULL values are radically different from RDF bnodes. Indeed a bnode is EQUAL to itself but different from any other bnode. This is because a RDF bnode represents the existence of an unknown value. >>> >>> --e. >>> >>>> Il giorno 12 ago 2019, alle ore 16:41, Diogo FC Patrao <djogopatrao@gmail.com> ha scritto: >>>> >>>> >>>> Vanity license plates in USA are strings, right? Then this problem would only happen if NULL='NULL', which is not. >>>> >>>> It could be that the private company stored 'NULL' instead of NULL to the unassigned tickets, but that's really bad coding/design (and easy to fix, I guess). >>>> >>>> Or maybe the DAO wrongly translate NULL to 'NULL' at some point. >>>> >>>> Cheers >>>> >>>> dfcp >>>> >>>> -- >>>> diogo patrão >>>> >>>> >>>> >>>> >>>> On Mon, Aug 12, 2019 at 11:11 AM Young,Jeff (OR) <jyoung@oclc.org> wrote: >>>> Here’s an example showing blank nodes being used to declare the place of birth is unknown in Wikidata: >>>> >>>> >>>> >>>> https://w.wiki/6$y >>>> >>>> >>>> >>>> In the UI, it is rendered like this: >>>> >>>> >>>> >>>> <image001.png> >>>> >>>> >>>> >>>> Jeff >>>> >>>> >>>> >>>> From: Daniel Hernandez <daniel@degu.cl> >>>> Date: Monday, August 12, 2019 at 9:42 AM >>>> To: "semantic-web@w3.org" <semantic-web@w3.org> >>>> Subject: [External] Re: The Joy of NULLs (not) >>>> Resent-From: <semantic-web@w3.org> >>>> Resent-Date: Monday, August 12, 2019 at 9:37 AM >>>> >>>> >>>> >>>> As Enrico pointed, blank nodes can be used to represent unknown values. >>>> An example of this use is Wikidata. I don't know another example. >>>> >>>> -- >>>> Daniel >>>> >>>> On Mon, 12 Aug 2019 07:36:41 +0000 >>>> Franconi Enrico <franconi@inf.unibz.it> wrote: >>>> >>>> > Mike, this could easily happen in an RDF world if you register a >>>> > vanity licence plate with anything starting with "_". Indeed, bnodes >>>> > would be the right way to represent unknown but existing plates. --e. >>>> > >>>> > Il giorno 11 ago 2019, alle ore 23:10, Michael F Uschold >>>> > <uschold@gmail.com<mailto:uschold@gmail.com>> ha scritto: >>>> > >>>> >> This is hilarious. It could never happen in an RDF world! No value, >>>> >> no triple. >>>> >> >>>> >> He tried to prank the DMV. Then his vanity license plate backfired >>>> >> big time. >>>> >> https://mashable.com/article/dmv-vanity-license-plate-def-con-backfire/<http://flip.it/NIk7FD> >>>> >>>> >>>> >>>> >>> > -- Hugh 023 8061 5652
Received on Sunday, 25 August 2019 10:02:48 UTC