I don't know anything about the 1969 paper, other than its existence.
In the 1970 paper, he claimed and demonstrated that for any collection of
nested relations, there is a collection of unnested relations that can
express the same information. I'm not sure whether "can express the same
information" is an isomorphism or not, but I think that is what you are
driving at. He didn't use the terms "nested" and "unnested", but again I'm
pretty sure we're talking about the same thing here.
He called the collection of relations with no nesting the "normal form" for
that data. This term later became "1st normal form", when other normal
forms were discovered. What he called a collection of relations is, I
think, what you and I call a "schema". It's clear from the paper that
normalization is not an absolute requirement for using the relational model
of data.
I read a lot between the lines about what Codd's intent might have been in
including and excluding various topics from the 1970 paper. This is thin
ice. It is fair to assume that he was quite judicious about what to leave
in and what to leave out. The paper covers an enormous amount of ground in
relatively few words.
Here, I think that it might have been quite a lot easier to build the first
relational database management system if one took the simplifying assumption
that all attributes would be "simple" datatypes. (There have been a lot of
arguments in c.d.t. about what makes a datatype "simple", but in the
context of the paper, the term should be clear enough.) I think that Codd
may have been raising the point about normalization in order to forestall
arguments against building the first RDBMS based on the complexity of
supporting nested relations.
His point is that even if you don't support nested relations, the schema
designer is not limited by that in the expressive power of the system.
Anyway, this is my take on it. I never actually read the 1970 paper until
after I started participating in c.d.t. You will remember a certain former
participant who said that she had read the paper over and over again, and
still didn't unerstand it. I was naive enough, at that time, to believe
that I could help explain what Codd might have meant, and this would be
helpful.
This is just my take on it. HTH.
Received on Wed Mar 12 2008 - 20:14:11 CET
