Re: Functional dependencies

Thanks !

-- 
Papastefanos Serafeim
serafeim at otenet dot gr

? "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> ?????? ??? ??????
news:ZJQWb.331$hs4.62526_at_phobos.telenet-ops.be...


> Papastefanos Serafeim wrote:

> > Let's say that I have a relation with

> > the attributes A,B,C,D,E.

> > Is there a standard way to find the

> > candidate keys of that relation if I

> > am given a set of functional

> > dependencies for those attributes,

> > like A->B, BC->E, ED->A ?

> > I don't want the answer, I want to

> > know if there is a standard way to

> > find it.


>

> Yes, there is. The algorithm is described on

>

> http://en.wikipedia.org/wiki/Database_normalization

>

> The page is a mess but you can find it under the header "Algorithm

> (deriving candidate keys from FDs)". Basically it amounts to using the

> following rule to derive smaller superkeys:

>

>     if    C is a superkey and the FD X->Y holds

>     then  (C - Y) + X is also a superkey

>

> The algorithm starts with a set singleton set that contains the superkey

> that contains all columns, and then applies this rule to derive smaller

> superkeys. If it finds a smaller superkey that is a strict subset then

> the original superkey is removed from the set. Once you obtain a

> superkey for which you cannot derive a superkey that is a strict subset,

> you have found a candidate key.

>

> Good luck,

>

> -- Jan Hidders

>