ER Model - Outline

8/13/2019 ER Model - Outline

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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

The Entity Relationship ModelOutline

Basis of an Entity-Relationship Diagram (ERD)

Entities, relationships and attributes

Multiplicity and participation constraints on

relationships

Keys, superkeys and primary keys

Cyclic relationships

Weak entity types

ISA relationships (specialisation/generalisation) An informal algorithm for constructing an ERD
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

The Entity Relationship Model

An example of a high-level database model

Useful for design before moving to a lower level

model (e.g. relational) Alternatives include

UML (Unified Modelling Language) and ODL (Object Description Language)
http://goforward/http://find/http://goback/


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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

The Components of the ER Model

Data structures entity types attributes relationship types
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DatabaseManagement

Peter Wood

Overview

EntitiesAttributes

Relationships

Weak Entity Types

ISA relationships



Integrity constraints primary keys for entity types and relationship types,

and multiplicity (cardinality) constraints for relationship

types
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DatabaseManagement

Peter Wood

Overview

EntitiesAttributes

Relationships

Weak Entity Types

ISA relationships



Integrity constraints primary keys for entity types and relationship types,

and multiplicity (cardinality) constraints for relationship

types

The ER model is only a partial data model, since ithas no standard manipulative part
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DatabaseManagement

Peter Wood

Overview

EntitiesAttributes

Relationships

Weak Entity Types

ISA relationships

The Main Advantages of ERDs

The diagrams (ERDs) associated with entity-relationship

models

are relatively simple

are user-friendly

can provide a unified view of data, which is

independent of anyimplementeddata model


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DatabaseManagement

Peter Wood

Overview

EntitiesAttributes

Relationships

Weak Entity Types

ISA relationships

The Building Blocks of an ERD

Entities and entity types (entity sets)

Relationships and relationship types Attributes, attribute values and domains
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Entities and Entity Types

Anentityis a thing that exists and can be uniquelyidentified.

Anentity type(orentity set) is a collection of similar

entities.

An entity type

is similar to a class in object-oriented languages

has associatedattributes, which represent properties

of the entities comprising the entity type is represented in an ERD by a rectangle
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Attributes

Attribute names(or simplyattributes) are properties of

entity types.

Each attribute usually has onlysimplevalues (e.g.

integers or character strings) represented by anovalin an ERD, with a line to the

rectangle representing its entity type

Sometimesmulti-valuedattributes (e.g. Phones) areallowed

represented by adouble ovalin an ERD
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of Entity Type and Attributes

PersonNI#

Name Address

Phones

Each Person has a single Name, Address and

National Insurance number (NI#)

Each Person can have many Phones
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Domain

Thedomainof an attribute of an entity type is the set of

constant values associated with that attribute.

Domains can be

atomicsuch as the domain of integers or the domain

of strings, or

set-valuedsuch as the domain of finite sets of

integers or of finite sets of strings.
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Relationship Types

Arelationship typeis an association between two or more

entity types.

Relationship types are represented in an ERD by

diamond shapes,

with lines to each of the rectangles representing

entity types involved in the association

Example of a relationship typeTeachesbetween entity

typeLecturerand entity typeStudent:

Lecturer Teaches Student
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

RelationshipsArelationshipis an instance of a relationship type, i.e., it

is the set of associations between theentitiescomprising

the entity types associated by the relationship type.

Above diagram isnotan ERD

Entities for one entity type on left (green)

Entities for another entity type on right (red)

Lines between entities show the relationship
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Multiplicity Constraints in Relationship Types

many-to-one(or one-to-many)

An Employee Works in one Department or a

Department has many Employees.

one-to-oneA Manager Heads one Department and vice versa.

many-to-many

A Lecturer Teaches many Students and a Student is

Taught by many Lecturers.

D b


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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of many-to-one relationship type

Employee Works-in Department

The arrowhead is drawn at the one end of the

relationship type

Each Employee Works-in one Department Each Department has many Employees working in it

D t bE l f l i hi
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of many-to-one relationship

DatabaseE l f l i hi
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of one-to-one relationship type

Manager Occupies Office

The arrowhead is drawn at both ends of the relationship

type

Each Manager Occupies one Office

Each Office has one Manager occupying it

DatabaseE l f t l ti hi
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of one-to-one relationship

DatabaseE l f t l ti hi t
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of many-to-many relationship type


No arrowheads are drawn

Each Lecturer Teaches many Students

Each Student is taught by many Lecturers

DatabaseE ample of man to man relationship
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of many-to-many relationship

DatabaseMultiple relationship types
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DatabaseManagement

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Multiple relationship typesThere may be more than one relationship type between

two entity types.

Lecturer

Teaches

Tutors

Student

Teaches relationship type is many-to-many

Tutors relationship type is one-to-many (from

Lecturer to Student)

DatabaseParticipation Constraints in Relationships
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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Participation Constraints in Relationships

optionalrelationship e.g., an Employee may or may not be assigned to a

Department Participation of entity (e.g., Employee) in relationship

said to bepartial This is the default for relationships in our notation (Sometimes indicated by cardinality constraint of 0..*)

mandatoryrelationship e.g., every Course must be Taught by at least one

Lecturer Participation of entity (e.g., Course) in relationship

said to betotal Indicated by double lines in our notation (Sometimes indicated by cardinality constraint of 1..*)

DatabaseExample of Participation Constraints
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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of Participation Constraints


Some Lecturers may not Teach any Students Each Studentmustbe taught by at least one

Lecturer

Lecturer Teaches0..*

Student1..*

DatabaseBinary Versus General Relationships
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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Binary Versus General Relationships

Supplier PartSupplies

Project

each supplier may supply different parts to different

projects

Supplies is an example of a ternaryrelationship

DatabaseMKeys and Superkeys
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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Keys and Superkeys

Definition of a superkey. A set of attributes of an entity

type is asuperkeyif for each entity, saye, over that type,

the set of attribute values of the attributes in the superkey

uniquelyidentifiese.

Definition of a key. Akeyfor an entity type is a superkey

which isminimal.

simplekeys are single attribute keys, such as Emp#

and NI#.

compositekeys are keys having more than one

attribute, such as {Dname, College} and {Pname,

Address}.

DatabaseM t


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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Definition of primary key of an entity type. Aprimary

keyis a key that has been designated as such by the

database designer.

The primary key guarantees logical access to every

entity.

Attributes that are members of the primary key of an

entity type areunderlinedin an ERD.

PersonNI#

Name Address

Phones

DatabaseManagementCyclic Relationship Types


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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Cyclic Relationship Types

Definition. Acyclic relationshiptype (also calledrecursive) is a relationship type between two occurrences

of the same entity type.

Examples:

Marriage between Person and itself. Parent-Child between Person and itself.

Part-Sub-Part between Part and itself.

With each entity type in a cyclic relationship type we

associate arole, represented by labels on lines in anERD.

DatabaseManagementExample of a Cyclic Relationship Type with


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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Example of a Cyclic Relationship Type with

Roles

PersonNI#

Name Address

Phones

Married

husband wife

Roles are specified by husband and wife labels.

DatabaseManagementRelationship Types with Attributes


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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Relationship Types with Attributes

Examples:

attribute Date associated with the relationship type

Married attribute Price associated with the relationship type

Supplies

Supplier PartSupplies

Price

Each Supplier Supplies a Part atacertainPrice.

DatabaseManagementWeak Entity Types


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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Weak Entity Types

Definition. A Weak Entity Type is an entity type that does

not have sufficient attributes to form a primary key.

The existence of a weak entity depends on the

existence of anidentifyingorownerentity type.

The relationship between them is called an

identifying(ID) relationship. The weak entity type must have a discriminator(one

or more attributes) for distinguishing among its

entities.

For example, in an employees database, Childentitiesexistonly if their corresponding Parent

employee entity exists.

DatabaseManagementWeak Entity Type and ID Relationship
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Management

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Weak Entity Type and ID Relationship

Parent

Has

Child

NI#

Pname Address

Age

Cname Age

DatabaseManagementWeak Entity Types in an ERD


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g

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

y yp

A weak entity type is identified by a double rectangle. The discriminator is underlined by a dashed line.

An identifying relationship is identified by a double

diamond.

The fact that the existence of the weak entityrequires the existence of an owner entity is captured

by the total participation of the weak entity type in the

relationship (double line).

Theprimary keyof a weak entity type is the

combination of the primary key of its owner type andits discriminator, e.g., (NI#, Cname) for Child.

DatabaseManagementISA Relationship Types
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g

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

p yp

When an entity type contains certain entities that

have special properties not shared by all entities, this

suggests two entity types should be created with an

ISA relationship type between them.

Also known as generalisation/specialisation.

E.g. An Employee ISA Person and a Student ISA

Person (seeISA relationship types)

If Employee ISA Person then Employee inheritsall

the attributes of Person.

DatabaseManagementISA Relationships


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Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

p

Person

Name Number

ISA

EmployeeSalary Student Degree

Attributes of Employee: Name, Number and Salary.

Attributes of Student: Name, Number and Degree.

DatabaseManagementInformal Algorithm for Constructing an ERD


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Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

g g

1. Identify the entity types (including weak entity types)

of the enterprise.

2. Draw some instances of the identified entity types.

3. Identify the relationships (including ISA and ID

relationships) of the enterprise.

4. Classify each relationship identified in step 3

according to its multiplicity, i.e. if it is a one-to-one,

many-to-one or many-to-many.5. Draw some instances of the identified relationships.

6. Draw an ERD with the entity types and the

relationships between them.

7. Identify the attributes of entity types and their

underlying domain

8. Identify a primary key for each entity type.

9. Draw some instances of attribute values of entities.

10. Add the attributes and keys to the ERD drawn in step

6.

DatabaseManagementERD Example
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Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Employee

NameNumber

ISA

Mechanic SalesmanDoes

RepairJobNumber

Description

Cost

Repairs Car

License

ModelYear

Manufacturer

Buys

Price

Date

Sells

Date

Price

Commission

Client ID

Name Phone

Address

buyerseller

DatabaseManagementFor more information
http://find/


37/38

Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationshipsSee, e.g.,

Chapter 12 of [CB10].

Chapter 7 of [SKS11].

Chapter 4 of [UW08].

DatabaseManagementT. Connolly and C. Begg.

D t b S t A P ti l A h t
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Peter Wood

Overview

Entities

Attributes

Relationships

Weak Entity Types

ISA relationships

Database Systems: A Practical Approach toDesign, Implementation, and Management.Addison-Wesley, fifth edition, 2010.

A. B. Silberschatz, H. F. Korth, andS. Sudarshan.Database System Concepts.McGraw-Hill, sixth edition, 2011.

J. D. Ullman and J. Widom.A First Course in Database Systems.Prentice Hall, third edition, 2008.
http://find/

Documents

ER Model - Outline