MIS 310 Lecture Notes - Lecture 1: Data Modeling
Document Summary
An entity class is a description of the structure and format of the occurrences of the entity. An entity instance is a specific occurrence of an entity within an entity class. The most important elements of e-r models are: Relationships the requirements have been gathered, they are transformed into an entity relationship (e-r) Entities have attributes that describe the entity"s characteristics: An identifier will identify a particular instance in the entity class: If the identifier is unique, the data value for the identifier must be unique for all instances. A composite identifier consists of two or more attributes. Entities can be associated with one another in relationships. Relationship degree defines the number of entity classes participating in the relationship: A single entity instance in one entity class is related to a single entity instance in another entity class. An employee may have no more than one locker; and. A locker may only be accessible by one employee.