TAILIEUCHUNG - Database Modeling & Design Fourth Edition- P16

Database Modeling & Design Fourth Edition- P16: 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. | 62 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling a Management view Figure Example of data modeling ing workstation or computer . Secretaries and managers are each allocated a desktop computer. A pool of desktops and workstations is maintained for potential allocation to new employees and for loans while an employee s computer is being repaired. Any employee may be married to another employee and we want to keep track of these relationships to avoid assigning an employee to be managed by his or her spouse. This view is illustrated in Figure . The third view shown in Figure involves the assignment of employees mainly engineers and technicians to projects. Employees may work on several projects at one time and each project could be headquartered at different locations cities . However each employee at a given location works on only one project at that location. Employee skills can be individually selected for a given project but no individual has a monopoly on skills projects or locations. Conceptual Data Modeling 63 manages b Employee view c Employee assignment view Figure continued 64 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling Global ER Schema A simple integration of the three views over the entity Employee defines results in the global ER schema diagram in Figure which becomes the basis for developing the normalized tables. Each relationship in the global schema is based upon a verifiable assertion about the actual data in the enterprise and analysis of those assertions leads to the transformation of these ER constructs into candidate SQL tables as Chapter 5 shows. Note that equivalent views and integration could be done for a UML conceptual model over the class Employee. We will use the ER model for the examples in the rest of this chapter however. The diagram shows examples of binary ternary and binary recursive relationships optional and mandatory existence in relationships and generalization with the .

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