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Re: Constraints and Functional Dependencies

From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 24 Feb 2007 23:14:33 GMT
Message-ID: <tF3Eh.1136285$R63.350991@pd7urf1no>


Walt wrote:

> "Marshall" <marshall.spight_at_gmail.com> wrote in message
> news:1172333601.148573.19370_at_v33g2000cwv.googlegroups.com...
> 

>>On Feb 24, 6:41 am, paul c <toledobythe..._at_oohay.ac> wrote:
>>
>>>Marshall wrote:
>>>
>>>>...
>>>>With such a system, a relation R with attribute a (which I will
>>>>write as R(a)) having a as a foreign key into S(b) is expressed
>>>>as follows:
>>>
>>>> forall R(a): exists S(b): a = b
>>>
>>>>So we can express foreign keys this way.
>>>>...
>>>
>>>I presume that if S had other attributes besides b, this definition
>>>would mean that b doesn't need to be a so-called primary key? (That
>>>would be okay with me.)
>>
> 
> This is a digression, rather than another nit pick.  The concept behind
> "primary key" adds nothing, mathematically, to the concept behind "candidate
> key".  Any one of the candidate keys could be used as a foreign key,  with
> equal effect.
> ...

I fail to see the digression. I asked the question in the first place to double-check whether Marshall had intended that b not necessarily be a key, even though he said he was defining how to express a foreign key.

My question has nothing to do with whether primary keys "add" anything to candidate keys, rather whether they add anything to "foreign references".

p Received on Sat Feb 24 2007 - 17:14:33 CST

Original text of this message

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