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Re: UNSAID drafted and mapped to SQL

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Fri, 10 Dec 2004 18:45:52 +0000
Message-ID: <41B9EEE0.7040805@hp.com>
To: "Eric Prud'hommeaux" <eric@w3.org>
CC: public-rdf-dawg@w3.org



Eric Prud'hommeaux wrote:
> In response to a thread on UNSAID in the comments list [1], I've
> drafted a section on UNSAID [2]. Andy asked about implementing it on
> an SQL-accessed triple store. I thought I'd share the answer with the
> WG. I test for default negation by doing an outer join to the triple
> in the positive and then constraining that part of that triple is
> NULL:
> 
> 
> CREATE TABLE holds (id INTEGER NOT NULL AUTO_INCREMENT,
>                     s VARCHAR(80), p VARCHAR(80), o VARCHAR(80),
>                     PRIMARY KEY(id));
> 
> -- _:a  foaf:name              "Alice" .
> -- _:a  foaf:page              <http://work.example/alice> .
> -- _:a  foaf:workplaceHomepage <http://work.example/alice> .
> -- _:b  foaf:name              "Bob" .
> -- _:c  foaf:name              "Eve" .
> -- _:c  foaf:page              <http://isp.example/eve> .
> -- _:c  foaf:workplaceHomepage <http://work.example/eve> .
> 
> INSERT INTO holds (s, p, o) VALUES ('_:a', 'foaf:name', '"Alice"');
> INSERT INTO holds (s, p, o) VALUES ('_:a', 'foaf:page', '<http://work.example/alice>');
> INSERT INTO holds (s, p, o) VALUES ('_:a', 'foaf:workplaceHomepage', '<http://work.example/alice>');
> INSERT INTO holds (s, p, o) VALUES ('_:b', 'foaf:name', '"Bob"');
> INSERT INTO holds (s, p, o) VALUES ('_:c', 'foaf:name', '"Eve"');
> INSERT INTO holds (s, p, o) VALUES ('_:c', 'foaf:page', '<http://isp.example/eve>');
> INSERT INTO holds (s, p, o) VALUES ('_:c', 'foaf:workplaceHomepage', '<http://work.example/eve>');
> 
> -- SELECT ?name
> --  WHERE  ( ?x foaf:name  ?name )
> --         UNSAID ( ?x  foaf:mbox  ?mbox ) 

That is query 1 from the doc but that runs on a different dataset.

I'm guessing but I think the query should be:

SELECT ?name
WHERE  ( ?x foaf:name  ?name )
        UNSAID ( ?x  foaf:page  ?mbox )

as the SQL below uses "foaf:page"

> 
> SELECT h1.o AS name
>   FROM holds AS h1
>   LEFT OUTER JOIN holds AS h2 ON h2.s=h1.s AND h2.p="foaf:page"
>  WHERE h1.p="foaf:name"
>    AND h2.o IS NULL
> 
> -- +-------+
> -- | name  |
> -- +-------+
> -- | "Bob" |
> -- +-------+

I get this result set.

If the order is reversed

i.e.

SELECT ?name
WHERE  UNSAID ( ?x  foaf:page  ?mbox )
        ( ?x foaf:name  ?name )

I get:
--------
| name |
========
--------

because the UNSAID has a more general pattern (?mbox is unbound so any person 
with foaf:page is rejected - that's all of them).

(My query processor executes query in the order written at the moment).

> 
> -- SELECT ?name ?homepage
> -- WHERE  ( ?x foaf:name  ?name )
> --        OPTIONAL { ( ?x  foaf:page  ?homepage ) 
> --            UNSAID ( ?x  foaf:workplaceHomepage  ?homepage ) }
> 
> SELECT h1.o AS name, h2.o AS homepage
>   FROM holds AS h1
>   LEFT OUTER JOIN holds AS h2 ON h2.s=h1.s AND h2.p="foaf:page"
>   LEFT OUTER JOIN holds AS h3 ON h3.s=h1.s AND h3.p="foaf:workplaceHomepage" AND h2.o=h3.o
>  WHERE h1.p="foaf:name"
>    AND h3.o IS NULL
> 
> -- +-------+--------------------------+
> -- | name  | homepage                 |
> -- +-------+--------------------------+
> -- | "Bob" | NULL                     |
> -- | "Eve" | <http://isp.example/eve> |
> -- +-------+--------------------------+

This is query 2 from the doc on the second dataset.

I get:

--------------------------------------
| name    | homepage                 |
======================================
| "Bob"   |                          |
| "Alice" |                          |
| "Eve"   | <http://isp.example/eve> |
--------------------------------------

which is different because I get a "Alice" row.

    ( ?x foaf:name  ?name )

gets all three people.


Then,
    _:a foaf:page <http://work.example/alice>
matches the first triple pattern of the optional:
    ( ?x  foaf:page  ?homepage )
with ?homepage = <http://work.example/alice>

Then for the UNSAID
    _:a foaf:workplaceHomepage  <http://work.example/alice>
exists, UNSAID is false, reject OPTIONAL.  But that leaves ?name = "Alice"

I expect:
( ?x foaf:name  ?name ) OPTIONAL anything
to pick ?name = "Alice" (and "Bob" and "Eve").

What ever is in "anything" does not influence outside the OPTIONAL.

What am I missing?

	Andy

> 
> 
> [1] http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2004Nov/thread.html#16
> [2] http://www.w3.org/2001/sw/DataAccess/rq23/#unsaid
Received on Friday, 10 December 2004 18:46:25 GMT

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