Re: Basic question?What 's the key if there 's no FD(Functional Dependencies)?
Date: Fri, 03 Nov 2006 04:01:22 GMT
Message-ID: <maz2h.506$7F3.332_at_newssvr14.news.prodigy.com>
"NENASHI, Tegiri" <tnmail42_at_gmail.com> wrote in message
news:1162516551.190354.4820_at_b28g2000cwb.googlegroups.com...
> Brian Selzer wrote:
>> "vldm10" <vldm10_at_yahoo.com> wrote in message
>> news:1162403098.627006.128180_at_i42g2000cwa.googlegroups.com...
>> > saturnlee_at_yahoo.com wrote:
>> >> What 's the key for it? ABC or nothing???
>> >
>> >
>> > ABC is not the key.
>> > Example: Let one partricular entity has A,B,C atributes
>> > and let these atributes take the following values:
>> >
>> > A B C
>> > -----------------------------
>> > 2 4 6
>> > 8 4 6
>> > 2 4 6
>> >
>> > ( ABC can be the key only in the trivial cases i.e if an entity has
>> > the atributes whose values never change)
>> >
>>
>> The above example is not a relation. A relation is a set of tuples.
>> Because it is a set, there can be no duplicates.
>>
>> A relation does not contain entities: it contains statements about
>> entities.
> > I thought in mathematica relation is subset of the product of two or > more sets. Or you mean 'relation' from first order logic ? It not > contains statements in logic also, its just a formula. Please explain. >
No, a relation is a subset of the product of zero or more sets. You can have a relation with zero attributes. You can have a relation with one attribute. RM relations, though similar, are not the same as mathematical relations. For one thing, an RM tuple is a set of named values, whereas a mathematical tuple is a list of values.
To avoid confusion, I guess I should have said that a relation contains a set of tuples which represent propositions. A propositions is a statement that is either true or false. The point I was trying to make is that a set of entities differs from a set of statements about entities.
> -- > > Tegi >
>> A key is a subset of the attributes of a relation schema such that the
>> projection over that subset onto any relation has the same cardinality of
>> that relation. A candidate key is a key that exhibits the additional
>> property that no proper subset of that key is also a key.
> Received on Fri Nov 03 2006 - 05:01:22 CET