Re: No exceptions?

J M Davitt wrote:
...
All the attributes in a relation comprise, at least, a superkey. The
set of attributes that qualify as a candidate key must hold unique values and no subset of those attributes must hold unique values. The
only relations that could have empty candidate keys are those with
empty headings, right?

Apologies - I'll try to read the rest tomorrow, but right now I'm tired (from watching a library video - "Guns, Germs and Steel" - the author seems to be the James Burke of biology, not that I know much about the latter). I thought a relation with any number of attributes could have only one value for them if it had an 'empty' set of candidate keys, eg. a relation that has only one tuple? (and a 'relvar' could have only one value at any instant in time)?

p
.

Relevant Pages

  • Re: No exceptions?
    ... All the attributes in a relation comprise, at least, a superkey. ... empty headings, right? ... way to ensure that a relvar is a singleton is to declare an empty ... I understand your point about candidate keys; ...
    (comp.databases.theory)
  • Re: No exceptions?
    ... set of attributes that qualify as a candidate key must hold unique ... empty headings, right? ... only one value for them if it had an 'empty' set of candidate keys, ... regardless of the number of attributes in the relvar. ...
    (comp.databases.theory)
  • Re: No exceptions?
    ... All the attributes in a relation comprise, at least, a superkey. ... set of attributes that qualify as a candidate key must hold unique values and no subset of those attributes must hold unique values. ... empty headings, right? ... I thought a relation with any number of attributes could have only one value for them if it had an 'empty' set of candidate keys, eg. a relation that has only one tuple? ...
    (comp.databases.theory)