Re: Functional Dependencies

From: Shayne G. Wright <swright_at_cityregina.com>
Date: 19 Apr 2001 14:50:21 -0600
Message-ID: <01c0c912$55b51920$2d00dd63@ts457030>

Hi Selina

In a FD (X -> Y), Y can only be a single value, which is functionally dependent on X

ex.

X Y

---

X1 Y1
X2 Y2
X1 Y1

In a MVD (X ->> Y), Y can be a fixed set of values

X Y

---

X1 {Y1, Y2}
X2 {Y3, Y4}
X1 {Y1, Y2}

A FD is essentially a stronger form of MVD (with a single value requirement, as opposed to a set). So, all FDs can be considered MVDs, if the entire set is considered to be a single value, but a MVD is not necessarily a FD. That is, in all cases X -> Y |= X ->> Y, but X ->> Y \> X -> Y, because:

(X ->> Y)

X Y

---

X1 {Y1, Y2}, is equivalent to

X Y

---

X1 Y1
X1 Y2, so obviously X \> Y

Take care,
Shayne

Selina Chang <wensui_at_prodigy.net> wrote in article <992u52$3geq$1_at_newssvr06-en0.news.prodigy.com>...
> Hi all,
>
> Does anyone know the difference between Functional Dependencies (FD) and
> Multivalued Dependencies (MVD)?
>
> Thank you very much.
> WS
>
>
>
Received on Thu Apr 19 2001 - 15:50:21 CDT