Database Reference
In-Depth Information
The reason for these problems can be explained as follows: The relation
Course
as
described, defines the following predicate:
•
Each course is to be accounted for (we say
Course
is under
contract).
•
Each course has a
CourseName
which is under contract.
•
Each course has a
CourseCred
which is under contract.
The predicate involves three distinct propositions. We are attempting to use the
timestamp attribute (
EffectiveDate)
to represent more than one proposition about the
attribute values. This, according to Date, is undesirable and violates the sixth normal form.
We now state Date's theorem for the sixth normal form (6NF):
Note
: 6NF as defined, essentially refines 5NF. It is therefore obvious from the
definition that a relation in 6NF is necessarily in 5NF also.
Let us now revisit the
Course
relation: With the introduction of the timestamp
attribute (
EffectiveDate
), and given the requirements of the relation, there is a non-trivial
JD that leads to the following projections:
CourseInTime
{CourseNo, EffectiveDate} PK [CourseNo,
EffectiveDate]
CourseNameInTime
{CourseNo, CourseName, EffectiveDate}
PK [CourseNo, EffectiveDate]
CourseCredInTime
{CourseNo, CourseCred, EffectiveDate}
PK [CourseNo, EffectiveDate]
Observe that the projection
CourseInTime
is strictly speaking, redundant, since it
can be obtained by a projection from either
CourseNameInTime
or
CourseCredInTime
.
However, in the interest of clarity and completeness, it has been included.
This work by C. J. Date represents a significant contribution to the field of database
systems, and will no doubt be a topical point of discussion in the future.
