What are Subtypes?
Introduction of the concept of subtype to enable the use of an existing attribute with a change of name, and a usage example where from a flights transaction we must record the departure airport and the arrival airport.
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Introduction of the concepts of: group of subtypes, subtype and supratype, consistency controls (referential integrity) and selection lists, table diagrams, attribute describing a transaction and Contextual Title property of an attribute. The case of usage studied in the multiple reference of a transaction to another. Other usage cases not studied are also presented.