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UML and Data Modeling: A Reconciliation by David Hay

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Figure 4-4 shows the way UML (and Information Engineering, for that matter) represent sub-types: These are shown outside the super-type box, attached via specialized arrows.

Figure 4-4: Sub-types in Conventional UML

The problem with this approach is that if you have many subtypes, your diagram quickly fills up, distracting from other, often more important, structures. Moreover, as the nesting gets deeper and deeper, it is progressively more difficult to see that an instance of the sub-type is in fact an instance of the super-type several levels up. The attributes and relationships ...

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