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Those tasks are illustrated in Figure 9.4, which shows how two export sche-
mas, ES
1
and ES
2
, can be integrated. The export schemas are expressed using
an E/R model, which is often used as the CDM in heterogeneous DBs. Sup-
pose that ES
1
and ES
2
have a conflict in that the student entity class of ES
1
represents all students, and the student entity class of ES
2
represents just post-
graduate students. In the conformance step, we thus rename the latter entity
class postgrad student so distinct entity classes now have distinct names. In the
merging step, we identify that postgrad student is a subclass of student and
introduce an is-a relationship between them. Finally, in the improvement
step, we identify that postgrad student has a redundant attribute id, which it
can inherit from student. Thus, we remove id from postgrad student, resulting
in a final global schema GS.
Performing those tasks requires that a sequence of transformations be
applied to schemas in such as way that the resulting schema is equivalent to
the original one. The sequence of transformations gives a pathway for trans-
lating data expressed in the constructs of the export schemas ES
1
and ES
2
into data expressed in the constructs of the global schema GS. The trans-
formations should be reversible in the sense that data and queries expressed
in the integrated schema GS must be translatable into data and queries
expressed in ES
1
and ES
2
. With such reversible transforms, it is possible to
automatically translate queries and data between DBs [7].
In [8] we propose a new formal definition for equivalence of E/R sche-
mas and give a set of primitive transformations that can be used to transform
E/R schemas into equivalent schemas. This set of primitive transformations
includes transformations for renaming entity classes, attributes, and relations
(rename
E
, rename
A
, rename
R
) and transformations for adding and deleting
entity classes, attributes, and relations (add
E
, del
E
, add
A
, del
A
, add
R
, del
R
). For
example, the conformance step in Figure 9.4 is performed by the following
primitive transformation:
rename
E
student,postgrad student
ID
ID
ID
ID
ES
1
Student
Student
Student
Student
Name
Name
Name
Name
GS
conform
merge
improve
þ
þ
þ
ID
ID
ID
Postgrad
student
Postgrad
student
Postgrad
student
ES
2
Student
Thesis
Thesis
Thesis
Thesis
Figure 9.4
Example of semantic schema integration.
