![]() We will be examining attributes in some detail. These characteristics that serve to describe entities and relationships are called attributes. For instance, we could have John enrolled in Introduction to Art on July 1, 2010 and was awarded an A+ on completion of that course. If we consider the enroll-in relationship, we know there is a date when the student enrolled in the course and a final grade that was awarded to the student when the course was completed. There is a course numbered 661 that is offered by the Art department and it is titled Introduction to Art. As we look back, we can see there is a student named John whose id number is 184 and his phone number is 283-4984.Ĭourses are shown with a course title, a course number, and belong to a department. For instance, the students in our example are described by the values for their name, id number, and phone number. Each relationship pairs one student with one course.įigure 7.3 Relationships shown as lines connecting entities AttributesĮntities and relationships have characteristics that describe them. ![]() ![]() John and Lee are enrolled in Introduction to Historyīelow, we depict four instances of the enroll-in relationship by drawing a line from a student to a course.John and Amelia are enrolled in Introduction to Art.Suppose we have the two courses and four students listed previously. Continuing with our educational example we know that students enroll in courses, and so this is one of the relationships we should know about. We can illustrate these in a number of ways:įigure 7.1 Entities shown as rows in tableįigure 7.2 Entities shown as sets RelationshipsĮntities can be related to one another and so we use relationships to describe how entities relate to one another. For example, consider four students (say John, Amelia, Lee, and April) and two courses (Introduction to Art and Introduction to History). Suppose we know of four student entities and two course entities. Typically, entities are the people, places, things, and events that we need to remember something about. For example, if one considers a system to support an educational environment, one is likely to decide that we need to keep track of students, instructors, courses, etc. EntitiesĮntities are the things we decide to keep track of. In the entity relationship approach to modeling, we analyze system requirements and classify our knowledge in terms of entities, relationships, and attributes. When the model is complete, we then create a physical model in some database management system (DBMS) typically a relational DBMS, or relational database management system (RDBMS). Įntity Relationship modeling is a process used to help us understand and document the informational requirements of a system as a logical or conceptual data model. Another later paper published in Software Pioneers: Contributions to Software Engineering (2002) is Entity-Relationship Modeling: Historical Events, Future Trends, and Lessons Learned and can be found at. This paper can be found at it is one of the most cited papers in the computer field, and has been considered one of the most influential papers in computer science. Peter Chen introduced entity relationship modeling in his paper The Entity-Relationship Model–Toward a Unified View of Data (ACM Transactions on Database Systems, Vol. In this chapter, we discuss the concepts required to develop an ERD and the Peter Chen notation. When designing a database, it is common practice for a database designer to develop an Entity Relationship model and to represent that model in a drawing, the entity relationship diagram (ERD).
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