Elements of the E-R Model

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Carmella Garrison
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1 Chapter 3: The Entity Relationship Model Agenda Basic Concepts of the E-R model (Entities, Attributes, Relationships) Basic Notations of the E-R model ER Model 1 Elements of the E-R Model E-R model was introduced by Peter Chen in 1976 Key elements: Entities Attributes Identifiers Relationships ER Model 2

2 Entities An entity is something that can be identified in the real world. Examples: Peter Johns, MIS, 2, May Carson, ACCT, Ray Charles, 1134, CS, Prof Entities of a given type are grouped into entity classes. ER Model 3 Entities (2) Entity Class STUDENT Name Major Year ID Alan John MIS May Carson ACCT FACULTY Name StaffID Dept Post Ray Charles CS Prof Entity Instances ER Model 4

3 Attributes Entities have attributes that describe the entity s characteristics. Types of attributes: single-valued, multivalued, composite. Examples are: Single-valued: Year 2 Multi-valued: {MIS, Marketing} Composite: (FirstName, LastName) ER Model 5 Relationships A relationship indicates how one or more entity classes interact with one another. Each entity plays a role in a relationship. Degree of the relationship: number of entities in a relationship. Understanding the relationship is important because it will affect how the database is constructed and used. ER Model 6

E-R Concepts entity type = entity class = entity entity instance = entity occurrence = record entity type: a set of objects in the real world with the same properties

4 E-R Concepts entity type = entity class = entity entity instance = entity occurrence = record entity type: a set of objects in the real world with the same properties (not the same values) with a physical or conceptual existence entity instance: each entity has its own values for each attribute ER Model 7 Sample E-R Diagram ER Model 8

5 Basic E-R Notation ER Model 9 Basic E-R Notation ER Model 10

6 Basic E-R Notation ER Model 11 E-R Model Constructs Strong versus Weak Entity Type Independent versus dependent entity. Identifying owner. Identifying relationship. Weak entity identifier is its partial identifier combined with that of its owner. ER Model 12

7 E-R Model Constructs Attribute - property or characteristic of an entity type Simple versus Composite Attribute Single-Valued versus Multivalued Attribute Stored versus Derived Attributes ER Model 13 E-R Model Constructs Identifier or Key - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type. Simple Key versus Composite Key Candidate Key ER Model 14

8 E-R Model Constructs Criteria for Selecting Identifiers Will not change in value. Will not be null. No intelligent identifiers (containing e.g. locations or people that might change) Substitute new, simple keys for long, composite keys. ER Model 15 Composite Attribute ER Model 16

9 Entity with a multivalued attribute (Skill) and derived attribute (Years_Employed) ER Model 17 Simple and composite key attributes (a) Simple key attribute ER Model 18

10 (b) Composite key attribute ER Model 19 Stored vs Derived Attributes Derived attribute: the value of an attribute is derived from something else FName LName Name SID DOB Majors STUDENT Age ER Model 20

11 Derived Attributes Example 1: Age = today date - Date_Birth Example 2: total number of staff can be calculated by counting the total number of staff entity instances Example 3: deposit = 2 * rent ER Model 21 How to Interpret this Model? Branch_No BRANCH Tel_No Total_ Branch Address Fax_No Street City ZipCode ER Model 22

12 Relationships Definition: a meaningful association among entity classes Meaningful implies that the relationship allows us to answer questions that could not be answered given only the entity classes In general, any number of entity classes may participate in a relationship ER Model 23 Relationships Relationship Type versus Instance Degree of a Relationship - number of entity types that participate in it. Unary (or Recursive) Relationship Bill-of-Materials ER Model 24

13 Relationship types and instances (a) Relationship type (Completes) ER Model 25 (b) Relationship instances ER Model 26

Relationships Degree of Relationship Unary Relationship Binary Relationship Ternary Relationship Attributes of Relationships Many-to-Many Associative Entities (Gerunds) All

14 Relationships Degree of Relationship Unary Relationship Binary Relationship Ternary Relationship Attributes of Relationships Many-to-Many Associative Entities (Gerunds) All relationships involved are many Result has independent meaning Gerund has one or more non-key attributes ER Model 27 Relationships of different degrees (a) Unary relationships ER Model 28

15 Role Names Unary relationship: the same entity class participates more than once in different rules supervisor EMP Supervises supervisee ER Model 29 (b) Binary relationships ER Model 30

16 Role Names Role names may also be used when two entity classes are associated through more than one relationships EMP manager member manages DEPT of staff works_for ER Model 31 (c ) Ternary relationships ER Model 32

17 Ternary Relationship Statement: venders can supply various parts to warehouses PART VENDOR supplies WAREHOUSE Ternary relationship 3 binary relationships ER Model 33 4-nary Relationship Solicitor Buyer arranges Bid Financial_ Institution A buyer, advised by a solicitor, and supported by a financial institution, places a bid for a property A simultaneous relationship among four instances ER Model 34

Database Principles: Fundamentals of Design, Implementation, and Management Tenth Edition. Chapter 7 Data Modeling with Entity Relationship Diagrams

Database Principles: Fundamentals of Design, Implementation, and Management Tenth Edition Chapter 7 Data Modeling with Entity Relationship Diagrams Objectives In this chapter, students will learn: The