Re: teaching relational basics to people, questions

On Dec 28 2009, 12:37 am, r..._at_raampje.lan (Reinier Post) wrote:
> Jan Hidders wrote:
> >On 21 dec, 23:07, r..._at_raampje.lan (Reinier Post) wrote:
>
> [...]
>
> >> [...]  It's hard fo me to tell, because I just did the
> >> required math and it turns out don't have $60 to spend on the book
> >> which contains the definition of 6NF required for this discussion,
> >> but if I can get by the Google Books preview, it appears to involve some
> >> degree of interpretation of domain values (as being totally ordered).
> >> Normal normal forms don't do this.
>
> >Wikipedia is your friend:
>
> I know ...
>
> ><http://en.wikipedia.org/wiki/Sixth_normal_form>
>
> ... that's why I'm so forgiving when it disappoints me, I guess.
> I even try to remedy its shortcomings every now and then, which
> is why you see half a definition on that page right now - I couldn't
> find the definition of "U-projection" to complete it, and apparently,
> nobody else can, which makes me wonder whether the subject should
> be on Wikipedia in the first place.  So please help out if you can.
>
> --
> Reinier

Google "U projection" relational.

msh.revues.org/docannexe9233.html
PROJECTIVE OPERATIONS ON RELATIONAL CONSTRAINTS Luigi BURIGANA1

"2. EXISTENTIAL AND UNIVERSAL PROJECTIONS Let A  O[W] be any constraint and U  W any set of variables. The eprojection

(existential projection) of A relative to U, denoted by A " U, is the set containing
each assignment in context (W,O) such that there is a way of modifying its subassignment
on U so that the resulting assignment is in A. The u-projection (universal
projection) of A relative to U, denoted by A # U, is the set containing each assignment
in context (W,O) such that for every modification of its sub- assignment on U,
the resulting assignment is in A. The two concepts are formally rendered by these
equations:

A " U ={p + q : p0 + q 2 A for some p0 2 O[U]} (2) A # U ={p + q : p0 + q 2 A for all p0 2 O[U]} (3)

in which + is catenation between local assignments and q stands for a generic member
of O[W \ U]."

Sorry, most symbols come out wrong if I copy and paste. Received on Mon Jan 04 2010 - 15:09:06 CET