Re: two nasty schemata, union types and surrogate keys

Brian wrote:

>On Sep 22, 5:27 pm, r..._at_raampje.lan (Reinier Post) wrote:
>> Brian wrote:
>> >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.
>>
>> But what *is* that proposition?  It might be
>>
>>   FIRTNAME LASTNAME is an employee at CORPORATION
>>
>> but it might just as well be
>>
>>   at some time in the past, it has been asserted that
>>   FIRTNAME LASTNAME is an employee at CORPORATION
>>
>> which is a closed world formulation of what is approximately
>> the open world counterpart of the first.
>
>I don't think it is. Assuming that the assertion was true at that
>time in the past, the proposition is temporally qualified, whereas the
>first isn't.

Under the first interpretation, the closed world assumption allows us to deduce that, if (Sally, Smith, Acme) is not in the table, Sally Smith is no Acme employee, while under the second interpretation, we don't know whether she is. So this 'closed-world assumption' doesn't actually limit our ability to leave the truth values of propositions undecided by what is the database. We just have to slightly modify which proposition is being expressed.

So the closed world assumption doesn't limit what can be stated about the world with base relations; it is only important when looking at logical inference, e.g. determining the interpretation of complex constraints.

-- 
Reinier