Databases Reference
In-Depth Information
R3: Each structured attribute in an entity (or a relationship) should
be unnested in such a way that only primitive attributes are visible in
the entity type. Intermediate names are missed.
·
R4: Each structured attribute in an entity is transformed into an
independent entity related to the mother-entity by a binary relation-
ship, generically named has-a. The cardinalities of this relation-
ship are those of the complex attribute in the original mother-entity.
·
Figure 13.16 illustrates an application of these rules. The Address attribute
has been unnested in the Editor entity. The Bookstores attributes have been
defined as an entity related to Editor.
Other intermediate mapping rules concern the homogenization of spe-
cialization hierarchies to make them complete and not overlapping.
Given that the conceptual schema is in a canonical form after these
transformations, the remaining mappings are direct mappings to the rela-
tional model.
Direct Mappings
Each entity in the conceptual schema is a candidate to become a 1NF rela-
tion in the logical schema. However, relationships and generalization hierar-
chies also have to be transformed. Transforming relationships can be done by
applying one of the following two rules:
R5: If the conceptual relationship is a binary relationship and one of
its roles has a 1-1 cardinality, it can be transformed into a reference
·
Editor
Name
Nb
Str
City
Rule R3
Editor
Name
Address:[Nb,Str,City]
Bookstores:Set of [Name,City]
1-N
Has-a
Rule R4
1-1
Bookstores
Name
City
Figure 13.16
Eliminating complex attributes.
