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Database Modeling & Design Fourth Edition- P19: 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. | 4.5 Entity Clustering for ER Models 77 Dominance grouping Abstraction grouping Constraint grouping Relationship grouping These grouping operations can be applied recursively or used in a variety of combinations to produce higher-level entity clusters that is clusters at any level of abstraction. An entity or entity cluster may be an c Constraint grouping d Relationship grouping Figure 4.9 Grouping operations 78 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling object that is subject to combinations with other objects to form the next higher level. That is entity clusters have the properties of entities and can have relationships with any other objects at any equal or lower level. The original relationships among entities are preserved after all grouping operations as illustrated in Figure 4.8. Dominant objects or entities normally become obvious from the ER diagram or the relationship definitions. Each dominant object is grouped with all its related nondominant objects to form a cluster. Weak entities can be attached to an entity to make a cluster. Multilevel data objects using abstractions such as generalization and aggregation can be grouped into an entity cluster. The supertype or aggregate entity name is used as the entity cluster name. Constraint-related objects that extend the ER model to incorporate integrity constraints such as the exclusive-OR can be grouped into an entity cluster. Additionally ternary or higher degree relationships potentially can be grouped into an entity cluster. The cluster represents the relationship as a whole. 4.5.3 Clustering Technique The grouping operations and their order of precedence determine the individual activities needed for clustering. We can now learn how to build a root entity cluster from the elementary entities and relationships defined in the ER modeling process. This technique assumes that a topdown analysis has been performed as part of the database requirement analysis and that the analysis has been .
