Schemas Integration - Information Systems Reengineering, Integration and Normalization - page 269

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Fig. 7.8 Merge EER models by generalizations

Substep 2.3. Merge entities by subtype relationship.
There is a one-to-one relationship between every instance of domain A and every
instance of domain X1 and between every instance of domain B and every instance
of domain X2. It is able to recover the instance of x, which is derived from either A
or B. The practical recovery search logic is that any element that does not exist in