Re: The Practical Benefits of the Relational Model

"Peter Koch Larsen" <pkl_at_mailme.dk> wrote in message news:<3d95577e$0$70305$edfadb0f_at_dspool01.news.tele.dk>...
> > For example, is a view
> >
> > select x+y, x-y from point
> >
> > updateable? If it is, then how about
>
> This view looks updateable for a DBMS with a sufficienty strong inference
> mechanism, but there are are subtle reasons it might not be so. As an
> example imagine inserting a tuple { 100, - 100}.
> This would have the solution x = 0,y = 100, but if x and y are represented
> with limited precision (e.g. standard reals as represented in the most
> normal IEEE format), the solution x = 3.0 E-21 and y = 100 and a plethora of
> other would be an equally good solution. Which of the two tuples should be
> inserted? What happens if e.g. the value { x = 0.0, y = 100.0} is already
> present - should a nonpresent solution be inserted?
> Even if we disregard these points as nitty-gritty, your next example raises
> other points...

To say it politely, finite precision float values (IEEE format or other) are "mathematically challenged". Yes one can do simple ariphmetics with them, but not real math. IEEE should put disclaimer on them: "This product is dangerous for a consumer, and can't be used for anything more serious than rendering 3D graphics". In databases, we can't even enforce constraints on real domains, let alone pretend equation solving. Therefore let just drop "real values" from any theoretical consideration altogether.   

> >
> > select arccos(x/(x*x+y*y)), sqrt(x*x+y*y) from point
> >
> > ?
>
> ...namely that solving for x,y might result in a solution, where the
> inserted tuple because of the infinite precision might not even be present
> in the result set. Again disregarding this as nitty-gritty, what about a
> query such as

The real issue here is that the mapping is not one-to-one. If we insert (x=0,y=1) point, then what angle it would be? Pi/2 or -3*Pi/2?  

> select uppercase(name) from person?

If there is uppercase(name)=name constraint on the name domain, then your view is clearly updateable. If there is lowecase(name)=name on the name domain, then it is still updateable. If there is no constraint on the name domain, then your view is not one-2-one, so that we have to add one more pseudocolumn to make it updateable:

select uppercase(name), isuppercase(name) from person

where isuppercase(name) maps each character of the "name" string into 0 or 1, depending if the ccharacter is upper or lower case. Received on Sat Sep 28 2002 - 20:38:12 CEST