Extended ER Model and other Modelling Languages - Lecture 2 - Introduction to Databases (1007156ANR)

2 December 2005

Introduction to Databases Extended ER Model and other Modelling Languages

Prof. Beat Signer

Department of Computer Science

Vrije Universiteit Brussel

http://www.beatsigner.com

http://www.beatsigner.com/

Beat Signer - Department of Computer Science - [email protected]

2 February 21, 2014

Extended Entity-Relationship (EER) Model

Over the years the ER model has been extended with

additional concepts to increase its expressiveness specialisation and generalisation (ISA relationship)

ISA constraints

aggregation


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Specialisation and Generalisation

An entitiy set can be further subgrouped

or specialised into distinctive entity sets an entity subset may for example have additional attributes

top-down approach

Parts of multiple entity sets may be generalised in a

higher level entity set bottom-up approach

Specialisations as well as generalisations are

represented by the same ISA relationship construct represented by a triangle in the EER model

A ISA B implies that every A entity is also a B entity

generalisation can be seen as the inversion of specialisation

ISA


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ISA Relationship

Persons

id name

Students

ISA

Teachers

PhD

StudentsProfessors

ISA

researchtopic

teachinghours

studentID

#projects

LivesAt Locations

address

0..*0..1

ISA


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ISA Relationship ...

The attributes of a higher level entity set (superclass) are

inherited by the lower level entity set (subclass) via the

ISA relationship e.g. students inherit the id attribute from persons

A subclass also inherits any relationship participation

from its superclass e.g. students, teachers etc. can participate in the LivesAt

relationship

ISA


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ISA Constraints

To model an application domain more precisely, specific

constraints can be defined over an ISA relationship

Membership constraints a condition-defined membership constraint assigns an entity to a

specific subclass based on one or more attribute values

a user-defined membership constraint can be used to manually assign entities to a specific subclass (manual classification)

Disjointness constraints an overlapping constraint over subclasses specifies that an entity

may belong to multiple subclasses

- e.g. a person may be a teacher as well as a student

- by default, subclasses participate in an overlapping constraint

ISA


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ISA Constraints ...

Disjointness constraints ... a disjoint constraint specifies that an entity can only belong to

exactly one subclass

- e.g. a teacher can be either a PhD student or a professor (but not both)

- expressed by writing the word disjoint next to the triangle

Completeness constraints a partial specialisation (generalisation) constraint specifies that

some superclass entities may not belong to any subclass

- default completeness constraint

a total specialisation (generalisation) constraint defines that each superclass entity must belong to a subclass

- e.g. each teacher must belong to the PhD Students or Professors subclass

(or to both of them)

- represented by a double line connection to the triangle (like total participation)

ISA


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ISA Constraints ...

disjointness and completeness constraints are independent of each other

ISA

Persons

id name

Students

ISA

Teachers

PhD

StudentsProfessors

ISA

researchtopic

teachinghours

studentID

#projects

LivesAt Locations

address

birthdate

Adults ISA

birthdate <

23.02.1996

disjoint

0..*0..1


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Aggregation

A limitation of the ER model is that relationships cannot

be modelled over other relationship sets

What if we would like to introduce a manager for each

(employee, company, duration) combination?

WorksFor CompaniesEmployees

id name name address

Durations

from to


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Aggregation ...

We cannot just create a

4-ary relationship WorksFor not all combinations have

a manager

Introduce a new

4-ary Manages

relationship problem: we

introduce some redundancy!

- (employee, company, duration)

are replicated in (manager,

employee, company, duration)



Durations

from to

Manages

Managers



Aggregation ...

An aggregation treats the

relationship as a higher

level entity can be used in another relationship



Durations

from to

Manages

Managers



Part-Of Hierarchies

PartOf

Tires

Cars

CPartOf TPartOf

Chassis

3..4

1..1

1..1

1..1

PartOf

particular product

general Part-Of

PartOf

Products0..1

superpart subpart

0..*



ER Design Issues

When do we model something as an attribute

and when as an entity set? there is no general answer and the choice depends on the

specific application domain to be modelled

When do we model something as a relationship set and

when as an entity set? a relationship set often corresponds to an action between entities

In general we should try to avoid higher level n-ary

relationship sets 3-ary relationship sets should be the maximum and even these

should be used carefully



Attribute or Entity?

Should the address in the example be modelled

as an attribute or as a separate entity?

This depends on the intended use of the address if we need an address for different entities (e.g. not just for

employees) it is better to model the address as separate entity

LocatedAt OfficesEmployees

id name

birthday

phone age #offices

address

street city

0..1 0..*



Attribute or Entity?

Should the duration be modelled as a from and to

attribute or as a separate entity?

Depends again on the use if we foresee that the same duration period is used by other entity

sets, it should be modelled as a separate entity



Durations

from to



N-ary or Binary Relationships?

Basically any n-ary relationship can be replaced

by a number of binary relationship sets "real" 3-ary relationships should not be replaced

- e.g. WorksFor presented earlier

the Parents relationship is a example of a non-real 3-ary relationship

- have to use null values if there is either no father or mother

- better to model it as two separate binary relationships

PersonsHasFather HasMotherchild child

father mother

mother

ParentsPersonschild

father



ER Design Principles

Make sure that you pay attention to all the entities, their

relationships and any other constraints mentioned in the

description (requirements) of a specific application

domain e.g. show cardinality constraints for all relationships

Avoid redundancy!

The resulting conceptual model should be as simple as

possible, but not simpler introduce only necessary entities and relationships

Some constraints (e.g. functional dependencies) cannot

be expressed in the ER model



ER Design Principles ...

In two weeks we will see how the conceptual ER model

can be mapped to a logical database schema

(e.g. relational database schema)

The resulting relational schema can be further analysed

by using functional dependencies and normalisation



ER Summary

Entities and attributes

Relationships cardinality constraints

participation constraints

Keys

Weak entities

ISA hierarchies

Aggregation



Classification and Typing

Aristotle (384 BC-322 BC) introduced

a biological classification (ontology) artificial classification system

organisms with the two subclasses plants and animals

animals further subclassified into animals with blood and animals without blood

animals also classified into walking animals, flying animals and swimming animals (habitat)

Characteristics (problems) of Aristotle's classification based on behaviour and not on similarities (non-evolutionary)

animals can belong to multiple classes

- e.g. a duck belongs to the walking animals as well as swimming animals class

Aristotle



Classification and Typing ...

Bertrand Russell defined the

type of a given predicate P as

the entity set spanned by P intensional notion of type

The notion of type for classi-

fying entities based on a set

of conditions was questionned

by Wittgenstein there exists no precise definition (type) for some general concepts

- e.g. no definition for the general concept of a game

We can have a definitional (e.g. intensional) or

prototypical definition of a type

Bertrand Russell Ludwig Wittgenstein



OM Model

Data model that integrates concepts from

the entity-relationship and object-oriented

data models based on objects, collections and associations

(binary collections)

- supports associations over associations

different types of collections

- sets, bags, rankings and sequences

Separates typing and classification (role modelling)

Operational part (algebra) over objects and collections

A family of object database platforms have been realised

based on the OM model (e.g. OMS Java)

Moira C. Norrie



OM Typing and Classification

{ p1,p2 } p1,p2,p3,p4 }

p1

p2

Classification

Entities

Objects

Roles

Collections

represented by

Semantic

Grouping

employee

designer

Typ

ing

Employees

Designers

Females

subtype

subcollection



OM Example (RSL Link Model)



The Unified Modelling Language (UML)

UML is a standard

developed by the

Object Manage-

ment Group (OMG) used to specify the

components of a software system

based on earlier work including the object-modelling technique (OMT) by Rumbaugh et al., the Booch method by Booch and object-oriented software engineering (OOSE) by Jacobson

UML supports different types of diagrams class diagrams, use case diagrams, package diagrams,...

Ivar Jacobson Grady Booch James Rumbaugh



UML Class Diagrams

A UML class diagram is similar to an ER model

A UML class is "like" an ER entity set contains attributes as well as methods

Employees

id name

birthday

phone age

address

employee

id

name

birthday

phone

address

age()

ER entity set UML class



UML Class Diagrams ...

UML relationship class with attributes can be used like

an ER aggregation in other UML relationships e.g. locatedAt in the example

Note that the position of the cardinality constraints are

switched in UML

ER relationships UML relationships

LocatedAt OfficesEmployeesemployee office

employee officeofficeemployee locatedAt

LocatedAt OfficesEmployeesemployee office

years

0..*0..1 0..* 0..1

employee officeofficeemployee

locatedAt

years



UML Class Diagrams ...

Teachers

PhD

StudentsProfessors

ISAdisjoint

teacher

phdStudent professor

Teachers

PhD

StudentsProfessors

ISA

teacher

phdStudent professor

overlapping

disjoint

ER specialisation UML specialisation



Modelling Exercises

Model the RSL link model in ER

Define an ER model for the following application You are going to develop a database for the Chamber of Com-

merce and Industry of Brussels (CVO) to manage information about language courses in Dutch, Spanish, French and Italian. The database should store information about the teachers and students as well as the individual language courses. A semester lasts for 13 weeks. Regular courses are scheduled for two sessions (each 3 hours) per week; advanced courses for three sessions (each 4 hours) per week. Each course has a difficulty level (module) of 1, 2 or 3 and costs a certain amount of money. A teacher may not give more than 4 courses and not teach in more than 2 languages. Each student has to do a final exam and will get a grade for the exam.



Homework

Study the following chapter of the

Database System Concepts book chapter 7

- sections 7.8-7.10

- Extended ER Model (EER)

- Unified Modelling Language (UML)



Exercise 2

Entity-Relationship (ER) model

Extended Entity-Relationship (EER) model



References

A. Silberschatz, H. Korth and S. Sudarshan,

Database System Concepts (Sixth Edition),

McGraw-Hill, 2010

M.C. Norrie, An Extended Entity-Relationship Approach

to Data Management in Object-Oriented Systems,

Proceedings of ER ’93, 12th International Conference on

Conceptual Modeling, Arlington, USA, December 1993

B. Signer and M.C. Norrie, As We May Link: A General

Metamodel for Hypermedia Systems, Proceedings of ER

2007, 26th International Conference on Conceptual

Modeling, Auckland, New Zealand, November 2007

2 December 2005

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Extended ER Model and other Modelling Languages - Lecture 2 - Introduction to Databases (1007156ANR)

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