Entity-Relationship Model

Entity Sets

Relationship Sets

Design Issues

Mapping Constraints

Keys

E-R Diagram

Extended E-R Features

Design of an E-R Database Schema

Reduction of an E-R Schema to Tables

A database can be modeled as:

a collection of entities,

relationship among entities.

An entity is an object that exists and is distinguishable from other objects.

Example:  specific person, company, event, plant

Entities have attributes

Example: people have names and addresses 

An entity set is a set of entities of the same type that share the same properties.

Example: set of all persons, companies, trees, holidays

Attributes

An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set.

Domain – the set of permitted values for each attribute

Attribute types:

Simple and composite attributes.

Single-valued and multi-valued attributes

4E.g. multivalued attribute: phone-numbers

Derived attributes

4Can be computed from other attributes

4E.g.  age, given date of birth