Re: Interesting article: In the Beginning: An RDBMS history

"dawn" <dawnwolthuis_at_gmail.com> wrote in message news:1144354101.878068.281070_at_z34g2000cwc.googlegroups.com...
>
> David Cressey wrote:
> > "x" <x_at_not-exists.org> wrote in message
news:e12djb$ha2$1_at_emma.aioe.org...
> > > > I don't think so. I think he was making the distinction between
> > > attributes
> > > > specified by name and attributes specified by position.
> > >
> > > What is the difference between a "name" and a "position" from a
> > > mathematically point of view ?
> >
> > I can't speak for Codd on this, and I don't choose to speak for myself.
>
> The only difference is the domain for the function, whether it is a set
> of counting numbers or a set of attribute names. If counting numbers,
> then there is an obvious order (function), represented as the order of
> a tuple.

This is not true.

If the numbers were explictily used as if they were names, in every place where a specific attribute is specified, your statement would be true. However, if attributes are expressed in the form of a list, as they are in mathematics when discussing relations, then the mapping between attributes and values is based on position in the list.

However, that was not Codd's point.

Codd's point was that users should not have to remember "names" like 23, 24, 25, ...etc. in order to specify attributes in a query.

>
> It amuses me when people make a big deal about there being no order of
> the attributes in a relation (which is then not strictly a relation as
> Codd pointed out). Given that attributes are specified to the system
> in some order (create table...) and output in some order, what do I
> care if under the covers it knows a mapping from the counting numbers
> to the attribute values or from attribute names to attribute values or
> both?

A lot of things amuse you.

In this case, your amusement stems from the fact that you view the text that represents a create table as being the create table itself. The source code for a Lisp program is a string of characters. But the actual Lisp program is a data structure in memory. The Lisp program can have a structure (like tree structure) that the text source can only express indirectly, through the use of parentheses. Received on Fri Apr 07 2006 - 00:05:10 CEST