Re: PIZZA time again :-)

David Cressey wrote:
> "Marshall Spight" <marshall.spight_at_gmail.com> wrote:
>
> The first question is, "meaningful in what context?"
> In terms of databases, there are at least two contexts: the context of the
> transaction that wrote the data, and the context of any transaction that
> reads the data. So the question is not "meaningful in use" but "meaningful
> in exchange".

Yes. Part of the problem with this thread is that we're partly examining
issues of the semantics of order, and we're partly modelling a domain that (I don't expect) any of us are familiar with, other than in the role of "eater."

> whereupon the customer says, "what I actually ordered was,
> > toppings([salami, mozarella, onions])."
>
> And the delivery man says, "well, it's the same thing.".

While certainly possible, this doesn't sound like a particularly real-world example to me. But this is comp.databases.theory, and not comp.databases.pizza, so perhaps I should confine my remarks to the first of those two domains. :-)

> Now, I don't care whether the cook always adds the ingredients in a set
> order or not. All I care about are the context of the order and the context
> of the delivery.
>
> So, to make a long story short, the question of "is it a list or a set"?
> boils down to "what are you going to do with the data"?
> This question is relevant to data modeling as well as to process modeling.

Agreed.

If, for a given application, we need to capture not only the elements of a collection, but also an order for them, then we have a list. If order is not required, then we have a set. Also, it is possible to model a list with a set, assuming you conform to certain constraints (index attribute starts at 0 and is consecutive) and vice versa, (ignore the list index and don't repeat list members.)

Marshall Received on Sat Sep 10 2005 - 16:34:36 CEST