LIS 558 - Entity-relationship model definitions

A thing that is of interest to the users and that has occurrences that can be uniquely identified one from another; e.g., bank accounts, books, employees, courses, etc.
Entity class
A group of entities of a given type.
Entity instance
A specific member of an entity class. For example, William Shakespeare might be an entity instance of the entity class Author.
A property of an entity; e.g., attributes of the entity class Course might be Course number, Section, Enrollment, Room number, etc.
Entity key (or identifier)
The attribute or set of attributes used to uniquely identify each entity instance of an entity class; e.g., Student number for the entity class Student.
An association or connection between two or more entities; e.g., employees work for departments, students take courses, libraries order books.

The main aid in finding a relationship is to note that a relationship is usually a verb.

A relationship between two entities is either 1-1 (one to one), 1-n (one to many) or m-n (many to many), depending on its maximum cardinality. An example of a 1-1 relationship might be between Student and Locker, if each student can have only one locker and students do not share lockers. If one of these conditions is broken, the relationship becomes 1-n; and, if both are broken, the relationship becomes m-n.

The membership of an entity class in a relationship is either mandatory (obligatory), in which case each entity instance in the class must be in the relationship, or optional (non-obligatory).

A relationship may be unary or recursive; this means that both the entities belong to the same entity class. For example, one author cites another; authors can even cite themselves.

Weak entity
An entity whose existence in the database is dependent on the existence of another entity. For example, the entity class Dependent in an employee data base is usually weak, because inclusion of information on a dependent depends on inclusion of information about the employee.

Participation of a weak entity in at least one relationship is mandatory.


Last updated July 5, 2001.
This page maintained by Prof. Tim Craven
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Faculty of Information and Media Studies
University of Western Ontario,
London, Ontario
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