Re: [External] Re: The Joy of NULLs (not)

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/ <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/ <https://www.wisnesky.net/>
Or other content I found on this work
     https://twitter.com/bblfish/status/1165195822625153024 <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 <mailto: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 <mailto: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$y <https://w.wiki/6$y>
>>>  
>>> 
>>> In the UI, it is rendered like this:
>>> 
>>>  
>>> 
>>> <image001.png>
>>> 
>>>  
>>> 
>>> Jeff
>>> 
>>>  
>>> 
>>> From: Daniel Hernandez <daniel@degu.cl <mailto:daniel@degu.cl>>
>>> Date: Monday, August 12, 2019 at 9:42 AM
>>> To: "semantic-web@w3.org <mailto:semantic-web@w3.org>" <semantic-web@w3.org <mailto:semantic-web@w3.org>>
>>> Subject: [External] Re: The Joy of NULLs (not)
>>> Resent-From: <semantic-web@w3.org <mailto: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 <mailto: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><mailto: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/ <https://mashable.com/article/dmv-vanity-license-plate-def-con-backfire/><http://flip.it/NIk7FD <http://flip.it/NIk7FD>>
>>> 
>>> 
>>> 
>>> 
>>