Chapter 11 & 12

Entity Relationship Modeling

(ER)

Thomas Connolly, Carolyn Begg, Database System, A Practical Approach to Design Implementation and

Management, 4th Edition, Addison Wesley

Pg 342~ 386

Learning Outcomes

• Entity types

• Relationship types

• Attributes

• Strong and weak entity type

• Attributes on relationship

• Structural constraints

Learning Outcomes

• Problem with ER Models

• Specialization / generalization

• Aggregation

• Composition

Characteristics of E-R Model

• Semantic data model• Express the logical properties of an

enterprise database• Design tools and documentation for data

base structure• No physical DBMS• Proposed by Dr. Peter Chen• Unified Modeling Language (UML)

Components of E-R Model

• Entity

• Attribute

• Key

• Relationship

• Structural constraints on relationship

Entity

• Definition– An object or concept

• Types– Strong entity (parent, owner, dominant)– Weak entity (child, dependent, or subordinate)

• Diagram Notation– Rectangular

Attribute• Domain

• Types– Single– Composite– Single-valued– Multi-valued– Derived

Key

• Candidate key

• Primary key

• Composite key

Relationship• Definition

– Association among entities

• Diagram Notation– Line – (relation or role name)

• Degree of a relationship– Number of participating entities– Types

• Unary (recursive relationship)• Binary• Ternary• Quaternary

Structural Constraints on Relationship

• Cardinality constraints– Zero-to-one 0..1– Zero-to-many 0..*– One-to-many 1..*– Many-to-many *..*

• Participation constraints– Total (mandatory, every one involved)– Partial (optional, only some involved)

• Improper relationship– Fan trap (ambiguous pathway)– Chasm trap (missing pathway)

Enhanced Entity-Relationship Model• Additional entity types

– Superclass: including one or more distinct subgroups in the data model– Subclass: a distinct subgroup of an entity type in the data model

• Attribute Inheritance– Specialization hierarchy (specialization: maximizing the differences

between members of an entity by identifying their distinguishing characteristics)

– Generalization hierarchy (generalization: minimizing the differences between entities by identifying their common characteristics)

– Is-A hierarchy • Constraints on specialization/generalization

– Participation (mandatory, optional)– Disjoint: disjoint (or), non-disjoint (and)

• Other– Aggregation (has a or is part of)– Composition (strong ownership of aggregation)

Design Steps

• Identify– Entity types, relationship types– Cardinality and participation constraints– Attributes– Keys– Specialize/generalize– EER diagram

• EER model example

• Create an enhanced ER diagram for a rental management using following entities:– Rental agency– Staff

• Part time• Full time

– Owner– Renter– Property

• Business• Home