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Database Modeling & Design Fourth Edition- P11

Database Modeling & Design Fourth Edition- P11 | 3.1 Class Diagrams 37 among multiple objects. For example a Car may belong to more than one Car Pool. The aggregation association is indicated with a hollow diamond attached to the class that holds the parts. Figure 3.1 indicates that a Car Pool aggregates Cars. Composition is another part of association in which the parts are strictly owned not shared. For example a Frame is part of a single Car. The notation for composition is an association adorned with a solid black diamond attached to the class that owns the parts. Figure 3.1 indicates that a Frame is part of the composition of a Car. Generalization is another common relationship. For example Sedan is a type of car. The Car class is more general than the Sedan class. Generalization is indicated by a solid line adorned with a hollow arrowhead pointing to the more general class. Figure 3.1 shows generalization from the Sedan class to the Car class. 3.1.2 Class Diagrams for Database Design The reader may be interested in the similarities and differences between UML class diagrams and ER models. Figures 3.2 through 3.5 parallel some of the figures in Chapter 2 allowing for easy comparison. We then turn our attention to capturing primary key information in Figure 3.6. We conclude this section with an example database schema of the music industry illustrated by Figures 3.7 through 3.10. Figure 3.2 illustrates UML constructs for relationships with various degrees of association and multiplicities. These examples are parallel to the ER models shown in Figure 2.2. You may refer back to Figure 2.2 if you wish to contrast UML constructs with ER constructs. Associations between classes may be reflexive binary or n-ary. Reflexive association is a term we are carrying over from ER modeling. It is not a term defined in UML although it is worth discussing. Reflexive association relates a class to itself. The reflexive association in Figure 3.2 means an Employee in the role of manager is associated with many managed Employees.