Re: No exceptions?
From: J M Davitt <jdavitt_at_aeneas.net>
Date: Sat, 01 Jul 2006 21:52:29 GMT
Message-ID: <x8Cpg.9037$vl5.5742_at_tornado.ohiordc.rr.com>
>
>
> That's what I mean, yes.
>
>
>
>
> I believe this is standard terminology. I may misremember or be
> confused; I'll check when I get back to my books on Monday.
>
>
>
>
> That makes little sense. A key is a set of attributes; "no attribute"
> (whatever kind of thing that may be) is never a member of such a set.
> Think sets and subsets!
>
>
> (An attribute is never a key; a key is a set of attributes, even if that
> set has cardinality one.) A set of attributes that satisfies the
> uniqueness property is a superkey; it is also a key if no proper subset
> of it also is a superkey. In a relation where the empty set is key, any
> non-empty set (including singleton sets) cannot be a key, because there
> is always a proper subset of it (namely the empty set) that is also a
> superkey.
Date: Sat, 01 Jul 2006 21:52:29 GMT
Message-ID: <x8Cpg.9037$vl5.5742_at_tornado.ohiordc.rr.com>
Jon Heggland wrote:
> J M Davitt wrote:
>
>>I think I got it. Am I correct in understanding that JH said, >>"If an empty candidate key is declared, any other keys are >>superkeys?"
>
>
> That's what I mean, yes.
>
>
>>I appreciate JH's preference for the terms key and auperkey and >>think the idea has some merit - everyone seems to understand the >>uniqueness of key values and his terminology neatly distinguishes >>keys which are and are not irreducible.
>
>
> I believe this is standard terminology. I may misremember or be
> confused; I'll check when I get back to my books on Monday.
>
>
>>I'm still pondering... >> >>Using JH's definitions: if an empty key is declared, any other keys >>are superkeys. This would be the case if one presumes that the "no >>attribute" is also a component of all those superkeys, right?
>
>
> That makes little sense. A key is a set of attributes; "no attribute"
> (whatever kind of thing that may be) is never a member of such a set.
> Think sets and subsets!
Absolutely; I should have said "empty attribute set".
>
>>And >>I don't understand why that must be so. Once a key without >>attributes is declared, the relation value can hold no more than one >>tuple - but I don't see why every other attribute cannot also be a >>key.
>
>
> (An attribute is never a key; a key is a set of attributes, even if that
> set has cardinality one.) A set of attributes that satisfies the
> uniqueness property is a superkey; it is also a key if no proper subset
> of it also is a superkey. In a relation where the empty set is key, any
> non-empty set (including singleton sets) cannot be a key, because there
> is always a proper subset of it (namely the empty set) that is also a
> superkey.
Thanks.
And thanks to Marshall, too.
[snip] Received on Sat Jul 01 2006 - 23:52:29 CEST