Re: teaching relational basics to people, questions

So what sense do make then of Date's example?

  "EMP {EMP#,DEPT#,SALARY} KEY {EMP#}
  (with the obvious semantics)"

You are confused. This is why I said to read some definitions.

If some relations and constraints meet certain criteria, we say they are in some certain normal form. The example has one relation and no (non-key-or-type) constraints. Since there are no non-key-or-type constraints, that collection of relations and constraints is in DK/NF. But the relation in the example has more than one non-key attribute. So that collection of relations and constraints is not in 6NF.

> we *assumed* DK/NF, so there cannot be any
> other constraints. 6NF should logically be subsumed, if not else then
> vacuously.

> Personally I find it very useful, at least as a mental aid, because it
> forces one to concentrate on the possibility of constraining your
> schema just a little bit further.

The sense of "constraint" in the first quote is the normal dbms one. But you seem to think 6NF is some kind of "constraint". We could say that in some sense
"a given set of relations and constraints rearranged into some normal form is more constrained than it was". But this is using the word "constraint" with a different meaning. This seems to be where you are confused.

It would probably help if you didn't use vague and/or suggestive (thus, meaningless) phrases like "logically subsumed". You do this a lot.
Force yourself to identify and refer to the things, notions and terms you find in definitions
(like, a collection of some relations and some constraints).

philip Received on Thu Nov 19 2009 - 04:56:48 CET