Re: two nasty schemata, union types and surrogate keys

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 22 Sep 2009 08:55:58 -0300
Message-ID: <4ab8bb46$0$23780$9a566e8b_at_news.aliant.net>



Roy Hann wrote:
> Brian wrote:
> 
> 

>>>>>Whatever. What is in the database is supposed to be true.
>>>
>>>>>Says who?
>>>
>>>>Everyone who advocates the closed world assumption.
>>>
>>>The closed world assumption doesn't tell you anything about what is
>>>actually in the database; it tells you how you are entitled to
>>>manipulate what you find in the database. I hinted at that in
>>>my first post when I wrote "All that matters is that we can make the
>>>inferences that we should be entitled to make from the assertions."
>>
>>You're wrong, of course, but don't take my word for it. According to
>>Date in /An Introduction to Database Systems, Eighth Edition/, page
>>161: 'the Closed World Assumption (also known as the Closed World
>>Interpretation) says that if an otherwise valid tuple--that is, one
>>that conforms to the relvar heading--does /not/ appear in the body of
>>the relvar, then we can assume the corresponding proposition is
>>false. In other words, the body of the relvar at any given time
>>contains /all/ and /only/ the tuples that correspond to true
>>propositions at that time.' So the closed world assumption tells us
>>that what is actually in the database is supposed to be true, while
>>what is not is supposed to be false.
> 
> Far be it from me to contradict Date, but there is no way on earth that
> he intended us to take that to mean "Garabage In, Garbage Out" doesn't
> apply to databases.

Um, note the word "assume" in the quoted passage above. He doesn't say it is false only that we can assume it is. Also note the phrase "supposed to be". He doesn't say it is, which we have no way of validating from the dbms, only that it is supposed to be.

> Date is just telling us the limits of how we are
> entitled to manipulate the database and--less directly--what the
> consequences of violating 5NF are.  

Very precisely and very carefully, I might add. Received on Tue Sep 22 2009 - 06:55:58 CDT

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