Re: No exceptions?

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sun, 02 Jul 2006 21:24:19 GMT
Message-ID: <7QWpg.5323$pu3.120642@ursa-nb00s0.nbnet.nb.ca>

paul c wrote:

> Bob Badour wrote:
>

>> paul c wrote:
>>
>>> paul c wrote:
>>>
>>>> Jon Heggland wrote:
>>>>
>>>>> paul c wrote:
>>>>>
>>>>>> Let me re-phrase my original question:  Is there a logical flaw in
>>>>>> substituting TABLE_DUM for x in the expression "x join y" when x 
>>>>>> is not
>>>>>> in the catalogue?
>>>>>>
>>>>>
>>>>> I don't know what precisely you mean by "logical flaw", so I'll pass
>>>>> judgement. If something should be substituted for x (a "default 
>>>>> value",
>>>>> so to speak), TABLE_DUM does seem the natural choice, though, as it
>>>>> corresponds to false/zero in some sense.
>>>>> ...
>>>
>>> I suppose my "logic", if I may call it that, went like this (at 
>>> evaluation time):
>>>
>>> 1) according to syntax, x must be a relation
>>
>> What about the relation 'x = true' ? Or rather 'x != false' ?
>>
>>
>>> 2) according to the catalogue, there are no attributes for a relation 
>>> named x, so given that the syntax insists x is a relation, it must be 
>>> the same relation as either TABLE_DEE or TABLE_DUM
>>
>> Or it could be a boolean expression. If one treats 'x = 1' as a 
>> relation to handle restrict, then naturally x by itself is a boolean 
>> assertion. Is it not?
>> ...

>
> For notational reasons only, if I give a name to the relation 'x = 1',
> say z, then I would say instead that if x is projected out of z, z is a
> boolean assertion. However, in my example 'x join y', I don't see that
> projection comes into play.
>
> Assuming a db language admits a statement like 'SELECT y WHERE x = 1;'
> and we treat the restriction 'x = 1' as a relation I think that is the
> same as "'x = 1' join y" and we have stated something different from 'x
> join y'.

Exactly, 'x join y' states 'SELECT y WHERE x;' or 'SELECT x WHERE y' instead.

> One practical implication of TTM's definitions that I can think of is
> that evaluation of 'x <and> y' is not possible if x and y each have an
> attribute with the same name but different types

Any two values of differing most-specific-types are unequal. Thus, if x and y each have an attribute z where the types are different, the join will have cardinality zero and will have an attribute z with the most specific supertype of the z attributes in x and y. This might be the universal supertype.

, in other words closure
> isn't possible,

It is possible. See above. Received on Sun Jul 02 2006 - 16:24:19 CDT