Conceptual / semantic modelling

1

Conceptual / semantic modelling

Conceptual / semantic modelling

2

Overview

design; conceptual design; ER model

• concepts;

• semantic aspects;

• problems;

• Enhanced ER (EER) modelling;

transforming an ER (EER) model into a relational model

Conceptual / semantic modelling

3

Design

you know the mechanics for building a database

system, but don’t know how to design it

Conceptual / semantic modelling

4

Design

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

Conceptual / semantic modelling

5

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

Conceptual / semantic modelling

6

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

Conceptual / semantic modelling

7

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

Conceptual / semantic modelling

8

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 ...

Conceptual / semantic modelling

9

ER diagram - entity (UML)

Student

Book-Loan

Student

studNo

Conceptual / semantic modelling

10

ER diagram - entity (before)

Student

Book-Loan

Customer

Account

Conceptual / semantic modelling

11

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!)

Conceptual / semantic modelling

12

ER diagram - attributes (UML)

Student

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

Conceptual / semantic modelling

13

ER diagram - attributes (before)

year month day

date-of-birthname sex age

personal-details

Student

address

no code city tel-no

course fees

Conceptual / semantic modelling

14

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

Conceptual / semantic modelling

15

ER diagram - binary relationships (UML)

Student

name dOB

Book

TitleAuthorEdition

Borrows

Department

name

Belongs To

Conceptual / semantic modelling

16

ER diagram - binary relationships (before)

Student

Book-Loan Department

has belongs to

Conceptual / semantic modelling

17

ER diagram - ternary relationship (UML)

Student

name dOB

Module

code

Lecturer

name

Teach

Conceptual / semantic modelling

18

ER diagram - recursive relationships (UML)

Represents

Representative

Represented

Student

name dOB

Conceptual / semantic modelling

19

ER diagram - more than one relationship between two entities

Staff

name dOB

Department

name

HeadofDepartment

Member

Manages

BelongsTo

Conceptual / semantic modelling

20

ER diagram - relationships with attributes (UML)

Staff

name dOB

Department

name

IsAffiliatedTo

dateposition

Conceptual / semantic modelling

21

ER diagram - relationships with attributes (before)

Staff Departmentis affiliated to

member of staff

date duration position

Conceptual / semantic modelling

22

Structural constraints on relationships

cardinality constraints multiplicity

• cardinality

• participation

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

Conceptual / semantic modelling

23

Structural constraints on relationships (UML)

Student

name dOB

Room

hallno

Tutor

name

Rents

Advises

Teaches

0..1 0..1

0..*

1

0..*

1..*

Conceptual / semantic modelling

24

ER diagram - structural constraints (before)

rented_house

tenant

rents

1

M

landlord

owns

M

N

house history

has

1

1

Conceptual / semantic modelling

25

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

Conceptual / semantic modelling

26

ER diagram - fan trap

Student

name dOB

Course

code

Project

name

Attends

1..*

1

Proposes

1..*

1

Conceptual / semantic modelling

27

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

Conceptual / semantic modelling

28

ER diagram - solution to a fan trap

Student

name dOB

Course

hallno

Project

name

Attends Takes

1..* 1..*

11

can you think of another solution?

Conceptual / semantic modelling

29

ER diagram - chasm trap

Project

name

Course

hallno

Student

namedOB

Proposes Takes

1..* 0..1

0..*1

Conceptual / semantic modelling

30

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

Conceptual / semantic modelling

31

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

Conceptual / semantic modelling

32

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

Conceptual / semantic modelling

33

Specialisation / generalisation

Staff

empNo {PK}namesalaryposition

Manager

experiencebonus

Developer

languageprojects

Support

area

SalesStaff

specialitysales

Conceptual / semantic modelling

34

Specialisation / generalisation

Staff

empNo {PK}namesalaryposition

Manager

experiencebonus

Developerlanguageprojects

Supportarea

SalesStaff

specialitysales

FullTime

notice

PartTime

workLoadjobLevel

Conceptual / semantic modelling

35

Constraints

overlapping of the child subclasses• disjoint - {OR}

• overlapping - {AND}

participation of the parent superclass• total - {Mandatory}

• partial - {Optional}

Conceptual / semantic modelling

36

Constraints

Staff

empNo {PK}namesalaryposition

Manager

experiencebonus

Developerlanguageprojects

Supportarea

SalesStaff

specialitysales

FullTime

notice

PartTime

workLoadjobLevel

{Optional, OR} {Mandatory, OR}

Conceptual / semantic modelling

37

Specialisation / generalisationmore than one parent

Manager

experiencebonus

SalesStaff

specialitysales

SalesManager

target

{Optional}

child inherits attributes of both parents

Conceptual / semantic modelling

38

Aggregation

Course

code

Module

code

Staff

name

Student

namedOB

Supports IsRegisteredFor

0..*

1..* 1..*

0..* 1..1

0..*

Conceptual / semantic modelling

39

Composition

Account

no

Agreement

textdate

Overdraft

dateValidUntilamount

Transaction

datetypeamount

1..1

1..1 1..*

1..1 1..1

0..*

Conceptual / semantic modelling

40

Summary

conceptual model ER Model

entity attribute relationship

• multiplicity

traps

EER model generalisation/specialisation

• constraints

aggregation