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Generalizations can be joined into generalization sets (trees of generalizations), which allow specifying additional properties on a group of generalizations - such as disjointness and completeness constraints.
Example of generalization in ER diagram.
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There are 4 combinations of these two settings. The “breadloaf” symbol joining generalizations into a tree shows these 4 variations (See the following figures).
Example of overlapping and partial specialization in ER diagram.
Example of overlapping and total specialization in ER diagram.
Example of disjoint and partial specialization in ER diagram.
Example of disjoint and total specialization in ER diagram.
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Examples of these two cases are shown in the following figures.
Example of attribute-based discriminator in ER diagram.
Example of a predicate-based discriminator in an ER diagram.
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