Re: Lucid statement of the MV vs RM position?

David Cressey wrote:
> In Ted Codd's 1970 paper, he points out that when a system of relations is
> devised to store a body of facts, there are other systems of relations that
> will express precisely the same body of facts. He then points out that
> within a group of such systems that are all logically equivalent, there
> will be (at least) one that contains no sets, lists, or RVAs as elements of
> a tuple.

...on two conditions:

"(1) The graph of interrelationships of the nonsimple domains is a collection of trees.
(2) No primary key has a component domain which is nonsimple.
The writer knows of no application which would require any relaxation of these conditions."

Other writers claim they *do* know of such applications, however, and have proposed (semi-)formal guidelines for identifying cases where RVAs may be appropriate.

> In c. 1992, C.J.Date redefined 1NF to permit RVAs as elements. Not
> everyone follows Date's definition. In particular, some introductory
> material to the RDM still teaches 1NF, 2NF, and 3NF as they were defined in
> the 1970s and 1980s.

*Most* do, I think---with the obvious weakness that they don't really define what atomic/simple domains are, or why it matters. Do you know of any textbooks apart from Date's that uses Date's definition? And of course, some introductory material teach yet another 1NF definition. Caveat lector, and think for yourself. (BTW, there isn't much disagreement on 2NF and 3NF, is there? Nor much interest, I think...)

> I hope the above summary can save a few iterations in the newsgroup.

:)

-- 
Jon