The Assignment:
Design a "good" entity-relationship diagram that describes the following objects in an university application: students, instructors, professors, and courses. Students are subdivided into graduate and undergraduate students. Students take a course in a particular semester and receive a grade for their performance. Sometimes students take the same course again in a different semester. There are no limits on how many courses a student can take, and on how many students completed a particular course. Each graduate student has exactly one advisor, who must be a professor, whereas each professor is allowed to be the advisor of at most 20 students. Courses have a unique course number and a course title. Students and professors have a name and a unique ssn; students additionally have a gpa; moreover, graduate students have a GRE-score, and undergraduate students have a single or multiple majors. Professors can be students and take courses, but graduate students cannot be undergraduate students.
a) Draw an entity-relationship model to represent the library.
b) Indicate the cardinalities for each relationship type;
c) Assign roles (role names) to each relationship if there are ambiguities
d) Use sub-types, if helpful to express constraints.
Purposes:
Ensure that you can analyze existing manual system and can draw Entity Relationship Diagrams (ERD) of any proposed system.
Before Starting:
An Entity Relationship Diagram
The entity relationship enables a software engineer to fully to fully specify the data objects that are input and output from a system the attributes that define the properties of these objects, and their relationships
Guidelines to make ERD
The ERD is constructed in an iterative manner. The following approach is taken.
1.During requirement elicitation, customer are asked to list the "things" that the application or business process addresses. These "things" evolve into a list of input and output data objects as well as external entities that produce or consume information.
2. Taking the objects one at a time, the analyst and customer define whether or not a connection exists between the data objects and other objects.
3.Whether a connection exists, the analyst and the customer create one ore more object/relationship pairs.
4. For each object/relationship pair, cardinality and modality are explored.
- Steps 2 through 4 are continued iteratively until all object/relationships have been defined. It is common to discover omissions as this process continues. New objects and relationships will invariably be added as the number of iterations grows.
- The attributes of each entity are defined.
- An entity relationship diagram is formalized and reviewed.
- Steps 1 through 7 are repeated until data modeling is complete.
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