Re: MV Keys (was: Key attributes with list values)

"Bob Hairgrove" <invalid_at_bigfoot.com> wrote in message news:ge0402529rs8ol422qng6n5sq1eic5utk5_at_4ax.com...
> On 26 Feb 2006 08:19:50 -0800, "Marshall Spight"
> <marshall.spight_at_gmail.com> wrote:
>
> >mAsterdam wrote:
> >> dawn wrote:
> >> > ... What does it mean that a list is a key? If
> >> > I change one value in the list, does that make it a new key? I would
> >> > think so.
> >>
> >> If I change the order of the items in the list,
> >> does that make it a new key? I would think so. (See below)
> >
> >Sure. If you change the order of the bits in the int,
> >that makes it a new key as well.
> >
> >If you make a logical change to the value, it's a different value.
> >(Likewise, if the logical value stays the same but something
> >else changes, it's the same value.)
>
> It's not quite that simple. As David pointed out, a "mushroom and
> onion pizza" is not necessarily different than an "onion and mushroom
> pizza". But the ordering of the bits in an integer is significant,
> whereas the ordering of ingredients for a pizza isn't (although I'd
> definitely want to make the crust before I put the tomato sauce on
> it<g>).
>

Thank you.

There is a way to implement sets as bit strings, where the bit number indicates which element of the universal set we are talking about and a value means inclusion for a 1 and exclusion for a zero. This is basically how Pascal implemented sets.

And, in this notation, a pizza with onion and mushroom has the same representation as a pizza with mushroom and onion. So, if the original order meant something, that information has been lost.

The way Pick (and I presume most of the MV family) represent lists is inherently ordered. And, whenever I ask Dawn, or any of the other Pickies who dorp in form time to time whether the order in a list conveys information or not, the answer is always the same: "the programmer knows what the data means".

The whole reason I got involved with databases, some twenty years ago, is to have a more formal and explicit handle on what the data means. As Marshall has pointed out, the semantics of the data are less than totally clear even in the RDM. But it's considerably closer to self describing data than just "ask the programmers". Received on Sun Feb 26 2006 - 21:43:46 CET