E-R Model: Overview of Elements, Diagrams, and Design Principles by Dr. Peter Chen, Slides of Database Management Systems (DBMS)

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Entity-Relationship Data Model

Presentation Overview

• Short historical overview
• Elements of E-R Model
• Basic organization & relationships in E-R

Model

• Design principles

Introduction

• Database Structure is often called Database Schema
• E-R model is graphical in nature, thus making it easy to analyze and observe relationship between data elements
• Most DBMS are based upon E-R model
• E-R model is not a good match for the sophisticated data structures required in today’s DBMS

Elements of E-R Model

• Data represented graphically via entity-

relationship diagram which contains 3 main element types:

• Entity sets
• Attributes
• Relationships

E-R Diagram

• Represents E-R elements by nodes of specific

shape to indicate kind

• Entity sets are represented by rectangles
• Attributes are shown as ovals
• Relationships correspond to diamonds
• Simple example from the book:

Simple illustration

E-R Relationships

• Suppose R is a relationship connecting entity sets E and F. Then: - If each member of E can be connected by R to at most one member of F , then we say that R is many-one from E to F. Note that in a many-one relationship from E to F, each entity in F can be connected to many members of E. - If R is both many-one from E to F and many-one from F to E, then we say that R is one-one. In a one-one relationship an entity of either entity set can be connected to at most one entity of the other set. - If R is neither many-one from E to F or from F to E, then we say R is many-many

Multi-way relationships

• There is a relationship Sequel-of between the entity set Movies and itself.
• To differentiate the two movies in a relationship, one line is labeled by the role Original and one by the role Sequel , indicating the original movie and its sequel, respectively.

Design Principles

• Faithfulness:
  • Design has to comply strictly with specifications
  • Logical attributes and relationships
• Avoid redundancy
• "Everything should be made as simple as possible, but not simpler.” Albert Einstein
• Choose right relationships
• Select right elements
  • Many choices exist between using attributes and using entity set/relationship combinations
  • An attribute is simpler to implement than either entity set or a relationship

Design Principals Cont.

• To replace an entity set by an attribute or attributes of several entity sets 3 conditions must be enforced: - All relationships in which entity set is involved must have arrows entering it. That is, it must be the “one” in many-one relationships, or its generalization for the case of multi-way relationships. - The attributes for E must collectively identify an entity. if there are several attributes, then no attribute must depend on the other attributes - No relationship involves E more than once