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.
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.
- Spring '14