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Figure 1 – uploaded by Bernhard Thalheim
Figure 1 Extended Entity-Relationship Diagram for Course Management new (abstract) entity type that is associated through binary relationship types to the components of the original type. In this case, identification of objects of the new type is either inherited from the component types or is provided through a new (surrogate) attribute. The first option results in the introduction of so-called weak types. The direct translation of these weak types to object-relational models must be combined with the introduction of rather complex constraint sets. Typically, this complexity can be avoided if the abstract entity type is mapped together with the new relationship types to a singleton object-relational type. This singleton type is also the result of a direct mapping of the original higher-order relationship type. The diagram c can | be enhanced by an explicit representation of cardinality and other constraints. If we use ee 7 7 ny a, /Mm Nn/\ ¥ ae ae i. ee, ae m ,; Mm - 4 4 4% °~+O0-2747 f \ re 7 7 7 7