CS 320: Database Management Systems

Entity-Relationship Model

Ge (Frank) Xia, CS, Lafayette College

Purpose of E/R Model

Entity Sets

E/R Diagrams


Example

Relationships

Example

Relationship Set

Example

Multiway Relationships

Example

A Typical Relationship Set

Bar Drinker
Beer
Joe's Bar
Sue's Bar
Sue's Bar
Joe's Bar
Joe's Bar
Joe's Bar
Sue's Bar
Sue's Bar
Ann
Ann
Ann
Bob
Bob
Bob
Carl
Carl
Bud
Miller
Bud
Pete’s Ale
Bud Lite
Miller
Miller
Bud Lite

Many-Many Relationships

In Pictures

Many-One Relationships

In Pictures

Example

One-One Relationships

In Pictures






Representing “Multiplicity”

Example

Example

In the E/R Diagram

Attributes on Relationships

Example

Equivalent Diagrams

Example

Roles

Example

Example

Subclass

Subclasses in E/R Diagrams

Example

E/R Vs. OO Subclasses

Example

Keys

Keys in E/R Diagrams

Example

Example

Weak Entity Sets

Example

In E/R Diagrams

Weak Entity-Set Rules

Design Techniques

Avoiding Redundancy

Example: Good

Example: Bad

Example: Bad

Entity Sets Vs. Attributes

Example: Good

Example: Bad

Example: Bad

Don’t Overuse Weak Entity Sets

When To use Weak Entity Sets?