Schemas Integration - Information Systems Reengineering, Integration and Normalization - page 271

Substep 2.5. Merge entities by categorization.
X provides a view to schema A and schema B. X1 is a union of A1 and A2. There
is a one-to-one mapping between every unique instance of domain A1 or A2 and
every instance of domain X1. Entity X1 dominates entity (A1, A2) ⃒ entity (A1,
A2) ≤ entity X1, entity X2 dominates entity B ⃒ entity B ≤ entity X2 to ensure that
there is no information loss during transformation. It is able to recover the instance
of x1, which can be derived from either A1 or A2 (Fig. 7.11 ).

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Fig. 7.11 Merge schemas into categorization