Re: Bags versus sets; are they needed?

mikharakiri_at_yahoo.com (Mikito Harakiri) wrote in message news:<bdf69bdf.0204021828.89945a6_at_posting.google.com>...
> Aggregation and projection commute on bags, but don't on sets. Given
> that aggregation has been added to relational theory as an
> afterthought, purists don't think that aggregation is significant
> enough to cast a doubt on sets.
Aggregation was part of Codds original language, if i do not remember incorrectly. That projections commute bags but not on sets is not a reason for bags in itself, is it?

>
> Here is another issue that makes me uncomfortable with sets. Consider
> the following example:
>
> First Name Last Name Weight
> ---------- --------- ------
> Sam Jones 135.88
> Kara Jones 135.88
>
> What is projection to <Last Name, Weight> is? Would it contain two
> records or one? Does the answer depend upon precision?
The answer depends on precision of course. If the two weights are different, they ARE different - even when they are displayed in the same way. The same thing could happen with strings. Suppose you have unicode strings but the system that displays the names only has latin fonts available. No projection should happen in this case either.

Kind regards
Peter Koch Larsen Received on Wed Apr 03 2002 - 14:42:12 CEST