Database Modeling & Design Fourth Edition- P6: Database technology has evolved rapidly in the three decades since the rise and eventual dominance of relational database systems. While many specialized database systems (object-oriented, spatial, multimedia, etc.) have found substantial user communities in the science and engineering fields, relational systems remain the dominant database technology for business enterprises. | The Entity-Relationship Model This chapter defines all the major entity-relationship ER concepts that can be applied to the conceptual data modeling phase of the database life cycle. In Section we will look at the simple level of ER modeling described in the original work by Chen and extended by others. The simple form of the ER model is used as the basis for effective communication with the end user about the conceptual database. Section presents the more advanced concepts although they are less generally accepted they are useful to describe certain semantics that cannot be constructed with the simple model. Fundamental ER Constructs Basic Objects Entities Relationships Attributes The basic ER model consists of three classes of objects entities relationships and attributes. Entities Entities are the principal data objects about which information is to be collected they usually denote a person place thing or event of informational interest. A particular occurrence of an entity is called an entity instance or sometimes an entity occurrence. In our example employee 13 14 CHAPTER 2 The Entity-Relationship Model Concept Representation Example Entity Weak entity Relationship Attribute identifier key descriptor nonkey multivalued descriptor complex attribute Employee Figure The basic ER model department division project skill and location are all examples of entities. For easy reference entity names will henceforth be capitalized throughout this text . Employee Department and so forth . The entity construct is a rectangle as depicted in Figure . The entity name is written inside the rectangle. Relationships Relationships represent real-world associations among one or more entities and as such have no physical or conceptual existence other than that which depends upon their entity associations. Relationships are described in terms of degree connectivity and existence. These terms are defined in the sections that follow. The most common meaning
