Re: Functional Dependencies > Uniqueness Constraints

Bob Badour wrote:
paul c wrote:

Marshall wrote:

... (And of course there must be a rule that
says every base table must have at least one functional
dependency in which the union of the determinant set
and the dependent set equals the set of attributes. (This
restriction is sufficient to ensure every base table is a
relation; is it necessary?))
...

I would say not necessary. If a table is a representation of a
relation, then I`d think that even if no rule is stated, by definition
the union of the attributes is a CK, eg., if there is no stated
determinant set, all the attributes are in the dependent set. I can`t
think why one would want to state this, shouldn`t a dbms assume itÉ

I think you have determinant and dependent reversed. The attributes of a
candidate key are the determinant set, and the remaining attributes are
each dependent attributes. Thus, if no other key is specified, all
attributes are in the determinant set and the set of dependent
attributes is empty.

What Marshall stated is an invariant of every relation for every
candidate key. In fact, it seems to me Marshall's statement is just a
restatement of candidate keys, but there could be subtleties I miss.

Yes, that's exactly what it is: a restatement of the requirement for
at least one candidate key, but stated in the terminology of
functional dependencies. The requirement comes from the
definition of set, of course, so any constraint of whatever
form that satisfies the definition of set (which is to say,
no duplicates) is sufficient.

I think the way I stated it is overly strong for what is
necessary to meet the definitional requirement. However
using any weaker requirement *for base tables* seems
like a really, really bad idea. On the other hand, for
a result set, the FDs are whatever they are.


Marshall

.

Relevant Pages

  • Re: Functional Dependencies > Uniqueness Constraints
    ... the union of the attributes is a CK, eg., if there is no stated ... determinant set, all the attributes are in the dependent set. ... candidate key are the determinant set, ...
    (comp.databases.theory)
  • Re: Functional Dependencies > Uniqueness Constraints
    ... and the dependent set equals the set of attributes. ... If a table is a representation of a relation, then I`d think that even if no rule is stated, by definition the union of the attributes is a CK, eg., if there is no stated determinant set, all the attributes are in the dependent set. ... The attributes of a candidate key are the determinant set, and the remaining attributes are each dependent attributes. ...
    (comp.databases.theory)
  • Re: Functional Dependencies > Uniqueness Constraints
    ... and the dependent set equals the set of attributes. ... If a table is a representation of a relation, then I`d think that even if no rule is stated, by definition the union of the attributes is a CK, eg., if there is no stated determinant set, all the attributes are in the dependent set. ... The attributes of a candidate key are the determinant set, and the remaining attributes are each dependent attributes. ...
    (comp.databases.theory)
  • Re: Functional Dependencies > Uniqueness Constraints
    ... and the dependent set equals the set of attributes. ... If a table is a representation of a relation, then I`d think that even if no rule is stated, by definition the union of the attributes is a CK, eg., if there is no stated determinant set, all the attributes are in the dependent set. ... The attributes of a candidate key are the determinant set, and the remaining attributes are each dependent attributes. ...
    (comp.databases.theory)
  • Re: Functional Dependencies > Uniqueness Constraints
    ... paul c wrote: ... the union of the attributes is a CK, eg., if there is no stated ... determinant set, all the attributes are in the dependent set. ...
    (comp.databases.theory)