# Re: Constraints and Functional Dependencies

From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Sat, 24 Feb 2007 23:26:51 +0100
Message-ID: <45e0bb3d\$0\$324\$e4fe514c_at_news.xs4all.nl>

paul c wrote:
> mAsterdam wrote:

```>> paul c wrote:
>>> mAsterdam wrote:
>>>> paul c 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:
>> (i)
>>>>>>   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.)

>>>> ...b should be a (candidate) key of S, ...
>>>
>>> My question is why?  Why should b be a key of S? You could answer
>>> "why not?" and I don't have a theoretical answer for that, other than
>>> the possibility, as Marshall hinted at, that requiring it to be a key
>>> isn't possible except on a relation-by-relation basis unless we just
>>> arbitrarily state it is always so, in which case my question remains
>>> "why?".
>>
>> Because (i) should, as Marshall stated, express
>> the notion of foreign key, specifically a foreign key to S.
>> I b is not a key of S, I don't see how it can.
>>
>> Cimode even gave a proof that it can't.
>> Don't you agree?
>> Is the proof flawed?
>> ...
```

>
> I don't know where to look for Cimode's proof

> but I think what Marshall
> defined is what I call a "reference", which seems more general than
> Codd's original foreign key, ie., doesn't exclude the possibility of the
> reference being a key in the referenced table, if the person who
> declares the reference so desires.

Yes, a reference (I like the term) is what the (i) defines when there b is not required to be a key of S. It is not what Marshall said he intended it to define.

> I take it that you want to read
> "foreign key" literally and insist that the reference involve a key
> defined for example, the way Marshall defined a key.

No need. I think it is clear.

> right, that's okay by me but I think "reference" encompasses "foreign
> key" in a way that never diminishes any table a designer who wants to be
> literal declares at the same as it allows the rest of the table definers
> much more leeway.
>
> The term "key" is a bastard in the first place. It seems to have been
> inherited from the times when hardware recognized keys by their physical
> position.

Etymological wars are not my idea of fun. Use RM.key if you need to.

```>> I think I do understand the need/wish to be able to have
>> this kind of constraint.
>> It would be stretching the concept of foreign key, no?
```

>
> As I suggested, I think that point of view is overly literal.
Received on Sat Feb 24 2007 - 23:26:51 CET

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