Re: What databases have taught me

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sat, 24 Jun 2006 02:35:08 GMT
Message-ID: <wx1ng.1377$pu3.36824@ursa-nb00s0.nbnet.nb.ca>

Aloha Kakuikanu wrote:

> Bob Badour wrote:
>

>>Consider:
>>
>>Ra = Rb AND x = 1 AND y = 1
>>
>>Where Ra and Rb are two relvars. Is that still a join? To what do x and
>>y refer?

>
>
> There is ambiguity caused by multiple equality signs. Did you mean
>
> (Ra = Rb) AND x = 1 AND y = 1
>
> or
>
> Ra := (Rb AND x = 1 AND y = 1)
>
> The first interpretation is absurd since there is no equality operator
> in relational algebra.

It exists in mine. It exists in the Date/Darwen one too. I disagree that it is ambiguous because obviously equality has a higher precedence than AND. Otherwise, R AND x = 1 AND y = 1 would end up as (R AND x) = (1 AND y) = 1, which I don't think you intended in any of your previous examples.

  In the second case there is no longer equality,
> but assignment := (and I still keep the redundant brackets for
> clarity).

I meant the first case, of course.

>>Consider:
>>
>>R AND x = 1 AND y = 1
>>
>>where R has no x attribute. Will that cause a compile-time error? Why or
>>why not?

>
> No. The cartesian product of
>
> R AND y = 1
>
> with
>
> x = 1
>
> is the EXTEND operator.

While that sort of thing might make sense for the internal algebra used by the dbms, I prefer something that yells loudly when I type anything that is almost certainly an error -- especially when the error would likely cause the query to run for orders of magnitude longer duration than expected.

> I realize that this kind of thinking would steer quite away from exact
> Tutorial D semantics. BTW, only now I noticed that Tutorial D is
> bastardised version of D&D "A" algebra. Of course, it may be argued
> that the connection Tutorial D <--> "A" is better designed than SQL
> <--> RA, but this parallel is kind of ironic.

I suggest there is a difference between the best formalism for a machine and the best formalism for a human, which is why compilers often start by converting programs into some sort of DAG as a first step. Received on Fri Jun 23 2006 - 21:35:08 CDT