Each table column represents a data field and each

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Unformatted text preview: pes of relationships among the data elements and map them properly in the form of a hierarchical tree structure when the database is first designed. For example, in the hierarchical database of Figure 16.7, an organization maintains a hierarchical database of all its employees. Employees are categorized by the department in which they work, and within a department they are categorized by their job function, such as managers, engineers, technicians, and support staff. If the personnel department has a shortage of support staff one day, producing a list of all support staff, to take a decision for using some support staff from other departments for this department, would not be directly possible. Instead, the support staff assigned to each department would have to be determined department-by-department. Network Databases A network database structure is an extension of the hierarchical database structure. In this model also, the data elements of a database are organized in the form of parent-child relationships, and all types of relationships among the data elements must be determined when the database is first designed. In a network database, however, a child data element can have more than one parent element or no parent at all. Moreover, in this type of database, the database management system permits the extraction of the needed information by beginning from any data element in the database structure instead of starting from the root data element. An example of a network database structure is shown in Figure 16.8. The example shows a network database of a college that maintains the relationships among the courses offered and the students enrolled for each course. As can be seen from the example, the parent and child elements can have many-to-many relationships in a network database structure. That is, each student may be enrolled in several courses, and each course may have a number of students enrolled for it. With this database structure used for this example, it is a course-wise students-wise courses report (a report showing all courses taken by each student). For example, it is easy to tell from the databa...
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This document was uploaded on 04/07/2014.

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