Conceptual / semantic modelling 1. 2 Overview design; conceptual design; ER model concepts; semantic...

Conceptual / semantic modelling

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Overview

design; conceptual design; ER model

• concepts;

• semantic aspects;

• problems;

• Enhanced ER (EER) modelling;

transforming an ER (EER) model into a relational model


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Design

you know the mechanics for building a database

system, but don’t know how to design it


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Design

requirements specification conceptual (semantic) design logical design physical design and implementation testing and validation maintenance


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Conceptual (semantic) design

constructing a model of the information used in an organisation

• independently of any targeted DBMS and of any physical considerations - high level of abstraction

• conceptual model - a more structured description (as opposed to a NL description)

• try to capture as much meaning as possible

model - constructed within a theory• terminology (ambiguity): model

most popular conceptual data model (theory)• ER (or E/R) model (theory) - Chen (1976)

• UML notation


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ER model

Concept Brief explanation Examples

entityany real world object(anything that has an individualexistence)

student, tutor, department,module, exam, project, plan,task

property something that describes an entity

(for student) name, address,age, fees paid, level of study;(for task) description,deadline;

relationshipa link that exist between twoentities

(student is) taught (module)by (tutor);(student) takes (exam);(project) includes (task)

attribute


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Concepts of the ER model

semantic, not formal• may be understood differently (by designers); results: different

models of the same real life system

• however, an ER model is a semi-formal description (see below)

not directly implementable• needs to be translated into an implementable model (e.g. the

relational model)

• why, then, doing it in the first instance?

symbolic level• formal description based on a set of informally defined concepts

• most popular: ER diagram


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Entity

entity type• a concept identified as having an independent existence

entity instance strong/regular entity type weak entity type

• its existence is dependent on some other entity types

examples• think ...


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ER diagram - entity (UML)

Student

Book-Loan

Student

studNo


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ER diagram - entity (before)

Student

Book-Loan

Customer

Account


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Attributes

a property/characteristic of an entity (or relationship) type instances of the same type have properties in common attribute domain

• should be included in a fully developed data model

attributes• simple / composite

• single-valued / multi-valued

• derived

• candidate key / primary key / composite key (note: no foreign key!)


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ER diagram - attributes (UML)

Student

name {PPK}dOB {PPK} day month yearsex/ageaddress no code city telNo[1..*]course/fees


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ER diagram - attributes (before)

year month day

date-of-birthname sex age

personal-details

Student

address

no code city tel-no

course fees


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Relationships

do not mistake with relation (in the relational model) relationship type

• a (meaningful) association between entity types

• participants - the entities involved in the relationship

relationship instance degree

• number of participating entities (types)

• binary

• ternary, quaternary …

• recursive


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ER diagram - binary relationships (UML)

Student

name dOB

Book

TitleAuthorEdition

Borrows

Department

name

Belongs To


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ER diagram - binary relationships (before)

Student

Book-Loan Department

has belongs to


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ER diagram - ternary relationship (UML)

Student

name dOB

Module

code

Lecturer

name

Teach


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ER diagram - recursive relationships (UML)

Represents

Representative

Represented

Student

name dOB


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ER diagram - more than one relationship between two entities

Staff

name dOB

Department

name

HeadofDepartment

Member

Manages

BelongsTo


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ER diagram - relationships with attributes (UML)

Staff

name dOB

Department

name

IsAffiliatedTo

dateposition


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ER diagram - relationships with attributes (before)

Staff Departmentis affiliated to

member of staff

date duration position


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Structural constraints on relationships

cardinality constraints multiplicity

• cardinality

• participation

relevant to all possibilities - i.e., not to a certain extension


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Structural constraints on relationships (UML)

Student

name dOB

Room

hallno

Tutor

name

Rents

Advises

Teaches

0..1 0..1

0..*

1

0..*

1..*


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ER diagram - structural constraints (before)

rented_house

tenant

rents

1

M

landlord

owns

M

N

house history

has

1

1


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Problems with ER models

connection traps• a link between two entities seems to have been represented via

another entity, but the actual model does not capture that

fan traps• symbolically: two (1:M) relationships fan out of an entity

• it is not always a trap

chasm traps• symbolically: a pathway representing a link contains a

relationship with a partial participation

a good understanding of the application is required


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ER diagram - fan trap

Student

name dOB

Course

code

Project

name

Attends

1..*

1

Proposes

1..*

1


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ER diagram - fan trap explanation

Fine Arts

Ann Boyle

Chris Evans

Monica Black

Colour and Depth

Mixed Media in Modern Art

Humour in Modern Art

has members proposes

these relationships are not represented


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ER diagram - solution to a fan trap

Student

name dOB

Course

hallno

Project

name

Attends Takes

1..* 1..*

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can you think of another solution?


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ER diagram - chasm trap

Project

name

Course

hallno

Student

namedOB

Proposes Takes

1..* 0..1

0..*1


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ER diagram - solution to a chasm trap

why not always represent all the relationships?

Project

name

Course

hallno

Student

namedOB

Proposes Takes

1..* 0..1

0..*1

Attends 1..*1


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The Enhanced ER model (EER)

additional semantic modelling concepts• for “richer” (more comprehensive) representations

type hierarchies specialisation / generalisation subclass / superclass attribute inheritance

aggregation hierarchies aggregation composition


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Specialisation / generalisation

equivalent names:• specialisation hierarchy, generalisation hierarchy or IS-A

hierarchy

specialisation• identifying classes among the instances of an entity type, by

identifying distinguishing properties (attributes);

• a top down approach

generalisation• grouping entity types into one single class, by disregarding

differentiating properties (attributes)

• a bottom up approach

attribute inheritance


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Staff

empNo {PK}namesalaryposition

Manager

experiencebonus

Developer

languageprojects

Support

area

SalesStaff

specialitysales


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Staff


Manager

experiencebonus

Developerlanguageprojects

Supportarea

SalesStaff

specialitysales

FullTime

notice

PartTime

workLoadjobLevel


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Constraints

overlapping of the child subclasses• disjoint - {OR}

• overlapping - {AND}

participation of the parent superclass• total - {Mandatory}

• partial - {Optional}


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Constraints

Staff


Manager

experiencebonus

Developerlanguageprojects

Supportarea

SalesStaff

specialitysales

FullTime

notice

PartTime

workLoadjobLevel

{Optional, OR} {Mandatory, OR}


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Specialisation / generalisationmore than one parent

Manager

experiencebonus

SalesStaff

specialitysales

SalesManager

target

{Optional}

child inherits attributes of both parents


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Aggregation

Course

code

Module

code

Staff

name

Student

namedOB

Supports IsRegisteredFor

0..*

1..* 1..*

0..* 1..1

0..*


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Composition

Account

no

Agreement

textdate

Overdraft

dateValidUntilamount

Transaction

datetypeamount

1..1

1..1 1..*

1..1 1..1

0..*


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Summary

conceptual model ER Model

entity attribute relationship

• multiplicity

traps

EER model generalisation/specialisation

• constraints

aggregation

Conceptual / semantic modelling 1. 2 Overview design; conceptual design; ER model concepts; semantic...

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