Let's analyze the result.
Figure 6.4: Projection of relation
onto a 4NF decomposition of
.
- This decomposition is not dependency preserving as it fails to
preserve
.
- Figure 6.4 (textbook 6.14) shows four relations that may
result from projecting a relation onto the four schemes of our decomposition.
- The restriction of
to
is
and some trivial
dependencies.
- We can see that
satisfies
as there are no pairs
with the same
value.
- Also,
satisfies all functional and multivalued dependencies since no two tuples have the same
value on any attribute.
- We can say the same for
and
.
- So our decomposed version satisfies all the dependencies in the
restriction of
.
- However, there is no relation
on
that satisfies
and decomposes into
and
.
-
Figure 6.5 (textbook 6.15) shows
.
- Relation
does not satisfy
.
- Any relation
containing
and satisfying
must
include the tuple
.
- However,
includes a tuple
that
is not in
.
- Thus our decomposition fails to detect a violation of
.
Figure 6.5: A relation
that does not satisfy
.