Re: Question on functional dependency

"G.L." <datapro01_at_yahoo.com> wrote in message news:dbe53197.0409211626.2f4a00bb_at_posting.google.com...
> I am reading the following:
>
> In a functional dependency X --> Y
>
> the value of Y is *determined* by the X component.
>
> and the value of X uniquely determines the value of Y.
>
>
> Then I read
>
> It is NOT true to say:
>
> " Since X functionally determines Y, if we know X we know Y."
> or
> "If X identifies Y then X identifies Y"
>
> (University of Virginia slide)
>
> I'm not clear on how X can uniquely determine Y but not functionally
> determine or identify
> Y
>
> Semantics?

Functional Dependency X->Y is a constraint such that the view image

select 1 from T as t1, T as t2
where t1.x=t2.x and t1.y!=t2.y

is empty.

Exercise 1. Prove Armstrong Transitivity property:

{}=select 1 from T as t1, T as t2
where t1.x=t2.x and t1.y!=t2.y

{}=select 1 from T as t1, T as t2
where t1.y=t2.y and t1.z!=t2.z

implies

{}=select 1 from T as t1, T as t2
where t1.x=t2.x and t1.z!=t2.z

Exercise 2. What view constraint specifies Multivalued Dependency? Exercise 3*. Prove Multivalued Dependency Transitivity Property.

(The star marks exercise that I didn't competed yet myself:-) Received on Wed Sep 22 2004 - 02:40:40 CEST