Database Processing Eighth Edition The Entity Relationship Model

Database Processing Eighth Edition The Entity. Relationship Model Chapter 3 David M. Kroenke 1 © 2002 by Prentice Hall

Data Modeling • Process of creating a logical representation of the structure of the database • The most important task in database development 2 © 2002 by Prentice Hall

Entity-Relationship Model (E -R Model) • An Entity-Relationship Model (E-R Model) consists of: – Entities – Attributes – Identifiers – Relationships 3 © 2002 by Prentice Hall

An Entity • An entity is an object that can be identified in the users’ work environment & that users want to track. • Entities of a given type are grouped into entity classes. 4 © 2002 by Prentice Hall

An Entity Example 5 © 2002 by Prentice Hall

Attributes • An attribute describes a characteristic of an entity • For example – An entity: Employee – Has attributes: • Employee. Name • Extension • Date. Of. Hire 6 © 2002 by Prentice Hall

Identifier • An identifier uniquely identifies a row in a table. • For an Employee, the Social. Security. Number may serve as the Indentifier. 7 © 2002 by Prentice Hall

Relationships • A relationship describes how one or more entities are related with each other. 8 © 2002 by Prentice Hall

Relationship Cardinality • Entity-Instance Participation in relationships is shown by – maximum cardinality – minimum cardinality 9 © 2002 by Prentice Hall

Maximum Cardinality • The maximum cardinality indicates/depicts the maximum number of instances involved in a relationship. • Alternatives include – 1: 1 (one-to-one) – 1: N (one-to-many) – N: M (many-to-many) 10 © 2002 by Prentice Hall

Relationship Examples Showing Maximum Cardinality Alternatives 11 © 2002 by Prentice Hall

Minimum Cardinality • The minimum cardinality indicates/depicts whether participation in the relationship is mandatory or optional. • Alternatives include – 0 (optional) – 1 (mandatory) 12 © 2002 by Prentice Hall

A Relationship Example Showing Minimum and Maximum Cardinality 13 © 2002 by Prentice Hall

A Recursive Relationship • A recursive relationship is when an entity has a relationship with itself. 14 © 2002 by Prentice Hall

Entity-Relationship Diagram (E-R Diagram) • An entity-relationship diagram (E-R Diagram) is a graphical representation of the E-R model using a set of ‘somewhat’ standardized conventions 15 © 2002 by Prentice Hall

An Entity-Relationship Diagram (E-R Diagram) Example 16 © 2002 by Prentice Hall

Weak Entity • A weak entity is an entity whose instance survival depends (logically) on an associated instance in another entity 17 © 2002 by Prentice Hall

Subtype Entities • Some entities may have many common attributes and a few unique attributes. • The common attributes may be grouped together in a supertype entity and the unique attributes may be grouped together in a subtype entity. 18 © 2002 by Prentice Hall

CLIENT with Subtype Entities 19 © 2002 by Prentice Hall

E-R Diagram Computer Assisted Software Engineering (CASE) Tools • Several Computer Assisted Software Engineering (CASE) Tools exist to help create E-R Diagrams and the resulting physical database elements. Products include: – IEW – IEF – DEFT – ER-WIN – Visio 20 © 2002 by Prentice Hall

Unified Modeling Language (UML) • The Unified Modeling Language (UML) is a set of structures and techniques for modeling and designing object-oriented programs (OOP) and applications. • A primary difference between UML & E-R Diagrams is that the UML representation includes information about object constraints and methods 21 © 2002 by Prentice Hall

E-R Diagram Example: Jefferson Dance Club 22 © 2002 by Prentice Hall

E-R Diagram Example: San Juan Charters 23 © 2002 by Prentice Hall

Database Processing Eighth Edition The Entity. Relationship Model Chapter 3 David M. Kroenke 24 © 2002 by Prentice Hall