Monday, December 15, 2008

Re: Semantics and schemas

Not an easy question to answer via email, but I will try..

All I meant when I said databases and/or XML don't have semantics is that the columns don't have any meaning that you might associated based on their "english" names.

Databases do have semantics--they have basically "look-up" semantics. Each tuple describes a complete world and you can do
selection/projection/join/count etc inference (queries). However, just because the database tuple says the person is "dead", you can't assume that the database can answer a query "is the person currently reading a paper?".

The more background knowledge you provide, the more deeper (i.e., non-lookup) inferences you can do. [A set of RDF triples provide no more inferential power than a set of relational tables conforming to a single schema. For example, a database tuple
<id=345, grade=A, gender=male> corresponds to the RDF triples t1--id--345; t1--grade--A; t1--gender--male.
It is the background knowledge--via RDF-Schema--that allows you to  do more inferences.]

--look at it another way. If you remember the magellan story-- after hearing that magellan is an explorer and he went around the world three times, a look-up inference can answer "Is magellan an explorer?" and "how many trips did he make?". A look-up inference however cannot answer the query "in which trip did magellan die?"--that requires more background knowledge. [Of course, if you are happy with just look-up inferences, then you don't need background knowledge. However, when you have multiple autonomous databases, and you wan't to "integrate" then, you in essense at least need a special type of background knowledge that at least maps the columns in both databases]

 if you want a really clean grasp of what it means to talk about inference and semantics, I would recommend that you take intro to AI ( )


On Sat, Dec 13, 2008 at 10:42 PM, Pierre Bucher <> wrote:
Dear Mr Kambhampati,

The answers to the question you put on the blog some time ago, about whether relational DB had semantics, made me doubt about the meaning of "semantics". I guess that XML Schema don't have semantics, because (1) we would not need RDF if it was the case (2) there is no concept of property and of inheritance in XML Schema, which seems to be the base of semantics. Then I would guess that relational DB don't have semantics either: they just have meta-data without meaning like XML and XML Schema. Is it true?

Moreover, is the fact that we have properties and relationships that gives RDF semantics? Then the concept of inheritance made possible by RDF Schema  would not give semantics, but just makes easier the processing of semantics (and actually we could do little without it). Is that right too?

Thank you.

Pierre Bucher.

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