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Database Modeling & Design Fourth Edition- P8

Database Modeling & Design Fourth Edition- P8 | 22 CHAPTER 2 The Entity-Relationship Model ER model constructs using the Chen notation ER model constructs using IDEF1X Bruc92 b One-to-one c One-to-many many side optional d One-to-many one side optional e Many-to-many b f Recursive binary relationship Figure 2.3 continued 2.2 Advanced ER Constructs 23 2.2 Advanced ER Constructs 2.2.1 Generalization Supertypes and Subtypes The original ER model has been effectively used for communicating fundamental data and relationship definitions with the end user for a long time. However using it to develop and integrate conceptual models with different end user views was severely limited until it could be extended to include database abstraction concepts such as generalization. The generalization relationship specifies that several types of entities with certain common attributes can be generalized into a higher-level entity type a generic or superclass entity more commonly known as a supertype entity. The lower levels of entities subtypes in a generalization hierarchy can be either disjoint or overlapping subsets of the supertype entity. As an example in Figure 2.4 the entity Employee is a higher-level a Generalization with disjoint subtypes b Generalization with overlapping subtypes and completeness constraint Figure 2.4 Supertypes and subtypes 24 CHAPTER 2 The Entity-Relationship Model abstraction of Manager Engineer Technician and Secretary all of which are disjoint types of Employee. The ER model construct for the generalization abstraction is the connection of a supertype entity with its subtypes using a circle and the subset symbol on the connecting lines from the circle to the subtype entities. The circle contains a letter specifying a disjointness constraint see the following discussion . Specialization the reverse of generalization is an inversion of the same concept it indicates that subtypes specialize the supertype. A supertype entity in one relationship may be a subtype entity in another relationship. When a .