Re: Polymorphism sucks [Was: Paradigms which way to go?]

Christian Brunschen wrote:
> In article <43328e59$1@xxxxxxxxxxxxxxxxxxxxxxx>,
> Dusty Bin <lixo@xxxxxxxx> wrote:
>
>>Christian Brunschen wrote:
>><snip>
>></snip>
>>
>>>So, without attempting to validate or invalidate anything else on any side
>>>of any argument, according to what I have quoted above, it should be
>>>fairly clear that the term 'Relational' in 'Relational Database' has
>>>_nothing_ to do with relationships between different tables:
>>
>>true
>>The term
>>
>>>'Relation' is instead a different, more precise term for the type of
>>>tables used in Relational databases. So, even a database with only a
>>>single table, is a relational database.
>>
>>The relationship is the relation between the columns of a table.
>
>
> Um, no.
>
> As defined in Codd's paper, a 'Relationship' is the same as a 'Relation',
> except for one thing. In a 'Relation', the order of the attributes
> matters, whereas their 'headings' are less important: one can, in a
> Relation, have multiple Attributes with the same Heading, as each
> Attribute is uniquely identified by its position within each Tuple in the
> Relation. In a Relationship, Attributes are unordered, but are identified
> by their Heading, which must of course now be unique.
>
> So the difference, in Codd's paper, is that a Relation is an unordered Set
> of Tuples, in which the ordering of the columns is significant but the
> names are less so; whereas a Relationship is an unordered Set of Tuples
> where each Column is uniquely identified by its Name 9rather than by its
> position).
>
> Or, to quote from Codd's paper,
>
> "In mathematical terms, a relationship is an equivalence class of those
> relations that are equivalent under the permutation of domains [ ... ] ."
>
>
> Of course, the term 'relationship' is another one of those that has become
> vastly overloaded over the years: For instance, it is used in
> 'Entity-Relationship modeling', to represent a connection between
> different entities, and since E-R modeling is often used when designing
> database schemas for relational databases, well, there's one source of
> confusion right there.
>
> But the term 'relationship' does not in fact as you are trying to state,
> com from there being a 'relation' between the different columns of a
> relation.
>
>
>>hth... Dusty
>
>
> Best wishes,
>
> // Christian Brunschen
>
Christian,
First of all, sorry to have confused the issue by using the term
'relationship', just a semantic error on my part, of course I meant
relation.
Unfortunately, I cannot lay my hands on Ted Codds paper right now, I
assume you mean "A Relational Model of Data for Large Shared Data
Banks", I've had it for over 30 years, and it is somewhere in my filing
system, and I'll re-append when I find it if it differs significantly
from below. I do however have Chris Dates "Introduction to Database
Systems", where Chris paraphrases Ted's definition, in section 11
references(p246 in the 4th edition).

"Given sets D1, D2, ..., Dn(not necessarily distinct), R is a relation
on those n sets if it is a set of n-tuples, each of which has it's first
element from D1, it second element from D2, and so on. The set Dj is
said to be the jth domain of R.) More concisely, R is a subset of the
Cartesian product of D1 X D2 X ... X Dn."

The only ordering I see in this definition is that the order of the
attributes must be in the same order of the domains, which by the way
are arbitrary. Simply said the attributes of the rows, must be in the
same domain as the header. To quote Chris Sonnacks sample table

NAME UID COLOR DATE TYPE
Alice - Blue 5-3-1948 BG45
Bob - Red 5-3-1948 XL220
Carol 3345 Red 12-7-1955 E30F
Dave 7709 Green 9-14-2000 -

Alice could not have a COLOR of 5-3-1948, and a DATE of blue.

For me, this table is a relation.

Best regards... Dusty
.

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