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Fig. 7.7 Merge EER models by union
In steps 2 and 3, the transformation processes are totally based on its precondition
without users' interference during the integration process.
Step 2. Merge entities.
Substep 2.1. Merge entities by union.
In this step, there is a one-to-one mapping between every instance of domain
A∪ and every instance of domain X, and vice versa (Fig. 7.7 ).
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Substep 2.2. Merge entities by generalization.
Case 1: Disjoint generalization—entities with the same attributes appear in two
schemas, but an instance of the first entity in one schema cannot appear as an in-
stance of the second entity in another schema. There is a one-to-one mapping be-
tween every unique instance of domain A or B and every unique instance of domain
X. This results to a one-to-one relationship between every instance of domain A or
domain B and every instance of domain X, and vice versa. It is able to recover the
instance of x, which is derived from either X1 or X2.
Case 2: Overlap generalization—entities with the same attributes appear in two
schemas, but an instance of the first entity in one schema can appear as an instance
of the second entity in another schema. There is a one-to-one mapping between
every unique instance of domains A and B and every unique instance of domain X.
This results in a one-to-one relationship between every instance of domains A and
B and every instance of domain X. It is able to recover the instance of x, which is
derived from either domain A or B (Fig. 7.8 ).
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