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Re: No exceptions?

From: J M Davitt <jdavitt_at_aeneas.net>
Date: Fri, 30 Jun 2006 05:37:38 GMT
Message-ID: <CM2pg.7937$vl5.1368@tornado.ohiordc.rr.com>


Bob Badour wrote:
> paul c wrote:
>

>> Bob Badour wrote:
>>
>>> paul c wrote:
>>>
>>>> J M Davitt wrote:
>> ...
>>
>>>>> It almost seems as though you want to declare an analogue for DUM,
>>>>> syntax-check some expressions, and add attributes to your relation
>>>>> with the confidence that your expressions are still correct.
>>>>
>>>> Not exactly how I thought of it, but I think that's fair, after all, 
>>>> one can add attributes, subject to one's external conception, to 
>>>> relation definitions that don't have empty headings, in fact not 
>>>> that the observation is of any use, that seems to be what happens 
>>>> when one defines a relation with one attribute.
>>>
>>> I suggest an empty candidate key in a relation with any number of 
>>> attributes is closer.
>>
>> I don't catch your drift.  If we are on the same page, then trying to 
>> equate a relation that has either no rows or one row with a relation 
>> whose name is mis-spelled is indeterminate.  If doing that is the same 
>> then I would have to give up on my original question.

>
> You suggested that adding an attribute to a degree zero relation is a
> degree one relation. However, whether one adds one attribute or an
> arbitrary number of attributes is less important than the fact that the
> candidate key will have an empty set of attributes.

Maybe a fine point, but...

All the attributes in a relation comprise, at least, a superkey. The set of attributes that qualify as a candidate key must hold unique values and no subset of those attributes must hold unique values. The only relations that could have empty candidate keys are those with empty headings, right? (I remember puzzling this when I first encountered DEE and DUM.) If the heading is not empty, must a non-empty subset of those attributes be declared as a key?

Because DEE and DUM have no attributes, they can hold no more than one tuple. If a relation heading contains even a single attribute, cardinality is boundless -- subject, of course, to any constraints on values and "physical" limits. In the single-attribute case, the superkey is irreducible and thus a candidate key. If more attributes are added, the arity of the candidate key might increase -- but it will never be less than one. After all: all relation variables have candidate keys. (I know I made a leap from relations to relation variables, but Paul did name these in his example.)

Have I missed something?

> Identifying the error condition is easy. The problem of identification
> only arises after you replace the error with DEE or DUM.
>
> Basically, you have a choice: your compiler can stop with an error or
> your compiler can allow anything by replacing non-existent relvars with
> an arbitrary constant.
>
> If one has a compiler that translates some user language into an
> internal language, one can catch most of the errors with it. Then, in
> the internal representation, once the compiler has established it is not
> an error, then it must be whatever it is.
Received on Fri Jun 30 2006 - 00:37:38 CDT

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