Information Technology Reference
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Tabl e 1 . Complex constructs: qualitative evaluation
Entity type
ET with a weakly specified subtype
→
supertype attribute
+ -
-
ET with an empty subtype
→
supertype attribute
+ +
≈
Relationship entity types
→ relationship type
? +
≈
+
Implicit Is-A rel.: materialization
→ explicit is-a
+ +
≈
+
Implicit Is-A rel.: upward inheritance
→
explicit is-a
- +
≈
+
Implicit Is-A rel.: downward inheritance
→
explicit is-a
≈
+ +
Relationship type
Inter-attribute functional dependencies
→
decomposition
- + +
Attribute
Complex attributes
→ entity type
- + +
One-component compound attribute
→
desagregation
+ + ?
Inter-ET functional dependencies
→ decomposition
- + +
Constraint
Decomposed existence constraints
→
merging
+
≈
+
Legend:
(-) Quality decrease
(+) Quality improvement
(
) Equivalent quality (?) Indeterminate quality change
(
≈
+
) Nearly equivalent quality with slight improvenent
≈
one of the source constructs (normally the lowest quality construct according to
its ranking in its equivalence class), which is (trivially) a semantics-preserving
transformation.
We classify the redundant constructs in two categories. The first category in-
cludes patterns in which some fact types of the application domain are expressed
more than once. The relationship/foreign key redundancy (figure 4
(a)
)
pattern is an example of this category. This pattern comprises an attribute that
references entities of another type (therefore acting as a foreign key) while a re-
lationship type already expresses such relationships explicitly. These constructs
are redundant and one of them must be removed. Since the foreign key suffers
from another problem (it appears as a
foreign construct
at the conceptual level
- see below), we remove it from the schema. A second example, not illustrated,
is that of attribute
Amount
of entity type
ORDER
, the values of which can
be computed from the values of attributes
OrderedQuantity
of
DETAIL
and
UnitPrice
of
PRODUCT
. The derived objects should be removed from the
schema in order to increase its simplicity and evolvability. The evaluation of
expressiveness is somehow less intuitive. As we removed the less expressive ob-
jects, we consider that the expressiveness of the whole construct has increased
too.
The second category includes constraints that can be formally inferred from
other constructs on the schema. We describe two examples.
