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Database Modeling & Design Fourth Edition- P12: Database technology has evolved rapidly in the three decades since the rise and eventual dominance of relational database systems. While many specialized database systems (object-oriented, spatial, multimedia, etc.) have found substantial user communities in the science and engineering fields, relational systems remain the dominant database technology for business enterprises. | 42 CHAPTER 3 The Unified Modeling Language UML Figure 3.5 UML n-ary relationship parallel to Figure 2.7 Figure 3.5 illustrates another example of a n-ary relationship. The nary relationship may be clarified by specifying roles next to the participating classes. A Student is an enrollee in a class associated with a given Room location scheduled Day and meeting Time. The concept of a primary key arises in the context of database design. Often each row of a table is uniquely identified by the values contained in one or more columns designated as the primary key. Objects in software are not typically identified in this fashion. As a result UML does not have an icon representing a primary key. However UML is extensible. The meaning of an element in UML may be extended Primary key as a stereotype Composition example with primary keys Figure 3.6 UML constructs illustrating primary keys 3.1 Class Diagrams 43 with a stereotype. Stereotypes are depicted with a short natural language word or phrase enclosed in guillemets and . We take advantage of this extensibility using a stereotype pk to designate primary key attributes. Figure 3.6 illustrates the stereotype mechanism. The vin attribute is specified as the primary key for Cars. This means that a given vin identifies a specific Car. A noteworthy rule of thumb for primary keys when a composition relationship exists the primary key of the part includes the primary key of the owning object. The second diagram in Figure 3.6 illustrates this point. 3.1.3 Example from the Music Industry Large database schemas may be introduced with high-level diagrams. Details can be broken out in additional diagrams. The overall goal is to present ideas in a clear organized fashion. UML offers notational variations and organizational mechanism. You will sometimes find that there are multiple ways of representing the same material in UML. The decisions you make with regard to your representation depend in part on your purpose for a given diagram.
