Course 6&7 ER Diagram

Course 6:Entity Relationship Diagrams

1. Basic Elements and Rules

Software Analysis & Design

Universeof Discourse

REQUIREMENTSCOLLECTION

AND ANALYSIS

FUNCTIONALANALYSIS

APPLICATIONPROGRAM

DESIGN

Description of requirements of users data modelling, process modelling

Data modelling is expressed using a high level model such as ENTITY-RELATIONSHIP (ER)

The ER Model can be represented pictorially (ER diagrams)

ER Model contains detailed descriptions of:

What are the entities and relationships in the enterprise?

What information about these entities and relationships should we store in the database?

What are the integrity constraints or business rules that hold?

Data modelling vs Process modelling

Process modelling (i.e. DFD) shows data stores, how, where, n when data r used or changed in an IS

Data modelling (i.e ER) shows d’definition, structure, n relationship within d’data

Y data model is d’most important part of the statement of SW requirement?

Characteristics of data captured during data modelling r crucial in d’design of DB, program, computer screen, n reports

Data rather than processes r d’most complex aspects of many modern IS so require a central role in structuring system requirement

D’characteristics of data (length, format, relationship) r reasonably permanen. D’paths of data flow r quite dynamic

Structural information about data is essensial for automatic generation of programs

Conceptual Data Modeling and the E-R Diagram Goal

Capture as much of the meaning of the data as possible

If you know the rules of normalization, referential integrity, foreign keys, etc., this is good but not as important now. It is much more important to get the organizational data model correct, i.e. to understand the actual data requirements for the organization.

Result A better design that is scalable and easier to

maintain

Introduction to Entity-Relationship (E-R) Modeling

Notation uses three main constructs Data entities Attributes Relationships

Entity-Relationship (E-R) Diagram A detailed, logical representation of the

entities, associations and data elements for an organization or business

10.610.6

Entity-Relationship (E-R) ModelingKey Terms

Entity A person, place, object, event or concept in the

user environment about which the organization wishes to maintain data

Represented by a rectangle in E-R diagrams Entity Type

A collection of entities that share common properties or characteristics

Attribute A named property or characteristic of an entity

that is of interest to an organization

10.710.7

Entity-Relationship (E-R) ModelingKey Terms

Candidate keys and identifiers Each entity type must have an attribute

or set of attributes that distinguishes one instance from other instances of the same type

Candidate key Attribute (or combination of attributes) that

uniquely identifies each instance of an entity type

Entity-Relationship (E-R) ModelingKey Terms

Identifier A candidate key that has been selected as the

unique identifying characteristic for an entity type

Selection rules for an identifier1. Choose a candidate key that will not change its

value2. Choose a candidate key that will never be null3. Avoid using intelligent keys4. Consider substituting single value surrogate

keys for large composite keys

Notation Guide

ENTITY TYPE

WEAK ENTITY TYPE

RELATIONSHIP TYPE

IDENTIFYING RELATIONSHIP TYPE

… Notation Guide

ATTRIBUTE

KEY ATTRIBUTE

MULTIVALUED ATTRIBUTE

DERIVED ATTRIBUTE

COMPOSITE ATTRIBUTE

__________

. . .

E1E1 R E2E2

E1E1 R E2E21 N

E2E2(min,max)

… Notation Guide

TOTAL PARTICIPATION OF E2 IN R

CARDINALITY RATIO 1:N FOR E1:E2 IN R

STRUCTURAL CONSTRAINT (min, max) ON PARTICIPATION OF E IN R (Alternative Notation)R

… Notation Guide

E1 IS A SUBCLASS OF E2

E1 and E2 ARE SUBCLASSES OF E3

Overlapping specialization

Disjoint specialization

o

d

E1E1 E2E2

E1E1

E2E2

E3E3

ER Diagram Basics

RelationshipRelationship

AttributesAttributes

EntityEntity

Product

Keeps

Store

descrip

qty

price

pname

manager

Locations

sname

Entity

Real-world object distinguishable from other objects (e.g a student, car, job, subject, building ...)

An entity is described using a set of attributes The same entity may have different prominence in

different UoDs– In the Company database, an employee’s car is of lesser importance– In the Department of Transportation’s registration database, cars may be the most important concept– In both cases, cars will be represented as entities; but with different levels of detail

Entity Sets

A collection of similar entities (e.g. all employees) All entities in an entity set have the same set of

attributes Each entity set has a key Each attribute has a domain Can map entity set to a relation easily

SSN NAME SAL

321-23-3241 Kim 23,000

645-56-7895 Jones 45,000

EMPLOYEES

Entity Type

Defines set of entities that have the same attributes (e.g. EMPLOYEE)

Each Entity Type is described by its NAME and attributes

The Entity Type describes the “Schema” or “Intension” for a set of entities

Collection of all entities of a particular entity type at a given point in time is called the “Entity Set” or “Extension” of an Entity Type

Entity Type and Entity Set are customarily referred to by the same name

EMPLOYEE

salSSN

name

Notation

Attributes

Key Attributes Value Sets of Attributes Null Valued Attributes Attribute Types

– Composite Vs. Simple Attributes– Single-valued Vs. Multi-valued Attributes– Derived Vs. Stored Attributes

NotationNotation

Key Attributes: Identifier

Key (or uniqueness) constraints are applied to entity types

Key attribute’s values are distinct for each individual entity in the entity set

A key attribute has its name underlined inside the oval

Key must hold for every possible extension of the entity type

Multiple keys are possible

EMPLOYEE

SSN

Null Valued Attributes

A particular entity may not have an applicable value for an attribute– Tertiary-Degree: Not applicable for a person with no university education– Home-Phone: Not known if it exists– Height: Not known at present time

Type of Null Values– Not Applicable– Unknown– Missing

Composite Vs. Simple Attributes

Composite attributes can be divided into smaller parts which represent simple attributes with independent meaning

Simple Attribute: Aircraft-Type Complex Attribute: Aircraft-Location

which is comprised of :Aircraft-LatitudeAircraft-LongitudeAircraft-Altitude

Notation

… … There is no formal concept of “compositeThere is no formal concept of “compositeattribute” in the relational modelattribute” in the relational model

Simple attributes can either be single-valuedor multi-valued Single-valued: Gender = F Notation

Multivalued: Degree = {BSc, MInfTech} Notation

… An “attribute” in the relational model is always single valued - Values are atomic!

Single Vs. Multivalued Attributes

Derived Vs. Stored Attributes

Some attribute values can be derived fromrelated attribute values: Age ® Date - B-day Y-Sal ® 12 * M-Sal

EMPLOYEE

M-sal

B-days Y-sal

Age

NotationNotation

Derived Vs. Stored Attributes

Some attribute values can be derived from attributed values of related entities

total-value ® sum (qty * price)

Order

Item price

qty

Total-Value

Representing Attributes

Parenthesis ( ) for composite attributes Brackets { } for multi-valued attributes

Assume a person can have more than one residence and each residence can have multiple telephones

{AddressPhone ({ Phone ( AreaCode,PhoneNum ) }, Address (StreetAddresss (Number, Street, AptNo),

City,State,PostalCode) ) }

Examples

Identify a few entity types, instances, attributes and candidate keys for: Case at DFD course

To be continued ....

Entity-Relationship (E-R) ModelingKey Terms

Relationship An association between the instances of

one or more entity types that is of interest to the organization

Association indicates that an event has occurred or that there is a natural link between entity types

Relationships are always labeled with verb phrases

Cardinality The number of instances of entity B that

can be associated with each instance of entity A

Minimum Cardinality The minimum number of instances of entity B

that may be associated with each instance of entity A

This is also called “modality”. Maximum Cardinality

The maximum number of instances of entity B that may be associated with each instance of entity A

Naming and Defining Relationships Relationship name is a verb phrase Avoid vague names Guidelines for defining relationships

Definition explains what action is being taken and why it is important

Give examples to clarify the action Optional participation should be explained Explain reasons for any explicit maximum

cardinality

Naming and Defining Relationships Guidelines for defining relationships

Explain any restrictions on participation in the relationship

Explain extent of the history that is kept in the relationship

Explain whether an entity instance involved in a relationship instance can transfer participation to another relationship instance

10.3110.31

Relationships

Relationship Types and Sets Relationship Degree Entity Roles and Recursive Relationships Relationship Constraints Attributes of Relationship Types

Relationship Types and Sets

A Relationship is an association among two or more entities (e.g John works in Pharmacy department)

A Relationship Type defines the relationship, and a Relationship Set represents a set of relationship instances

A Relationship Type thus defines the structure of the Relationship Set Relationship Type and corresponding Set are customarily referred to by the same name

Relationship Degree

The degree of a relationship type is the number of participating entity types– 2 entities: Binary Relationship 3 entities: Ternary Relationship n entities: N-ary Relationship– Same entity type could participate in multiple relationship types

PartPart

SupplierSupplier SupplySupply ProjectProject

EmployeesEmployees

DepartmentsDepartments

Works_InWorks_In

Assigned_toAssigned_to

TernaryTernary

MultipleMultiple

BinaryBinary

Entity Roles

Each entity type thatparticipates in a relationshiptype plays a particular rolein the relationship type

The role name signifies therole that a participatingentity from the entity typeplays in each relationshipinstance, i.e. it explains whatthe relationship means

Employees

Works_InWorks_In

Departments

employeremployer

workerworker

RoleRoleNamesNames

Recursive Relationships

Same entity type can participate more than once in the same relationship type under different “roles”

Such relationships are called“Recursive Relationships”

EmployeesEmployees

SupervisorSupervisor SubordinateSubordinateRecursiveRecursiveRelationshipRelationship

SupervisionSupervision

Relationship Constraints

What are Relationship Constraints ? Constraints on relationships are determined

by the UoD, which these relationships are describing

Constraints on the relationship type limit the possible combination of entities that may participate in the corresponding relationship set

Kinds of Constraints

What kind of constraints can be defined in the ER Model?

Cardinality Constraints Participation ConstraintsTogether called “Structural Constraints”

Constraints are represented byConstraints are represented byspecific notation in the ER diagramspecific notation in the ER diagram

The “Cardinality Ratio” for a binary relationship specifies the number of relationship instances that an entity can participate in– Works-In is a binary relationship– Participating entities are DEPARTMENT : EMPLOYEE– One department can have Many employees - Cardinality Ratio is 1 : N

Employees

Works_InWorks_In

Departments

Possible Cardinality Ratios

Possible Cardinality Ratios

1–to-1 (1 : 1)– Both entities can participate in only one relationship instance

1-to-Many, Many-to-1(1 : N, N : 1)– One entity can participate in many relationship instances

Many-to-Many (N: M)– Both entities can participate in many relationship instance

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.1- to - Many1- to - Many

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Many - to - Many - to - 11

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1-to-11-to-1

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.Many-to-ManyMany-to-Many

Example Cardinality Constraints

How many Employees can work in a Department? One employee can work in only one departmentHow many Employees can be employed by a Department? One department can employ many employeesHow many managers can a department have? One department can have only one managerHow many departments can an employee manage? One employee can have manage only one department

Representing Cardinality

One employee can work in only one departmentOne department can employ many employeesOne department can have only one manager

One employee can manage only one department

N 1

1 1

Employees

Works_InWorks_In

DepartmentDepartmentss

ManagesManages

Existence Dependency

Existence dependency indicates whether the existence of an entity depends on its relationship to another entity via the relationship type– Every employee must work for a department - EMPLOYEE is existentially dependent on DEPARTMENT via the Works In relationship type

EmployeesEmployees

Works_InWorks_In

DepartmentDepartmentss

Kinds of participating constraints

TOTAL Participation (Existence Dependency)Constraint : Every employee must work for a department

PARTIAL ParticipationConstraint : Not every employee is a manager

Representing Participation

Every employee must work for a departmentEvery department must have a managerEvery department must have employees

Not every employee is a manager

N 1

1 1

EmployeesEmployees

Works_In

DepartmentDepartmentss

Manages

To be continued..