Database Reference
In-Depth Information
Note
: The first join produces
SupplierSchedule
plus additional
spurious
tuples. The
effect of the second join is to eliminate the spurious tuples. To put it into perspective,
SupplierSchedule
is subject to a (time independent)
3-decomposable (3D) constraint
, namely:
This is an example of a
join dependency (JD) constraint
.
4.10.1 Definition of Join Dependency
A join dependency (JD) constraint may be defined as follows:
Relations that are in 4NF, but not in 5NF (such as
SupplierSchedule
) suffer from
duplication, which in turn leads to modification anomalies. These problems are directly
related to the presence of the JD constraint(s) in such relations. Fagin's theorem for
5NF relations provides the solution.
4.10.2 Fagin's Theorem
Fagin's theorem for the fifth normal form (5NF) states:
Now examine relation
SupplierSchedule. SupplierSchedule
is not in 5NF because
it has a JD (i.e. the JD constraint) that is not a consequence of its candidate key. In other
words,
SupplierSchedule
can be decomposed, but this is not implied by its candidate key
[Supl#, Item#, Proj#].
