R McFadyen 7-192.2911 Chapter 7 Conceptual Data Modeling.

7-192.2911R McFadyen

Chapter 7Chapter 7

Conceptual Data ModelingConceptual Data Modeling


TopicsTopics

topics:

– Developing a conceptual data model from use cases

– UML notations for conceptual data modeling

– UML class diagram bears a resemblance to an Entity Relationship Diagram (see 92.3914 for ERDs)


TopicsTopics

UML:

– attributes

– association, aggregation, and generalization

– multiplicities, roles, …


What Is a Conceptual Data Model?What Is a Conceptual Data Model?

A detailed model that shows the overall structure of organizational data; it is independent of any database management system or other implementation considerations.

Represented by UML class diagrams

Sources: • Current knowledge• Existing system• System documents / reports• Interviews• Use cases


Use cases are key inputs to conceptual data modeling.


Conceptual Data Model ElementsConceptual Data Model Elements

ClassesAttributes IdentifiersAssociations, aggregations, compositionsGeneralizationsTime dimensions Integrity rulesSecurity controls


What Is an Object?What Is an Object?

An entity that encapsulates data and behavior

Examples: product, employee, order, line item

Class – a set of objects that share the same attributes, operations, relationships, and semantics (abstract)

Instance – a single object (concrete)


Instances

We use a rectangular shape to represent a class. It may be shown with one, two or three compartments

attributes

operations

Name of class


Types of AttributesTypes of Attributes

Simple attributes – contain single data item

Identifiers – unique key value

Multivalued attributes – contain multiple values simultaneously

Composite attributes – group of related attributes


Stereotypes in Class DiagramsStereotypes in Class Diagrams

• Stereotype – a construct that extends the UML vocabulary

• Identifier stereotype <<PK>> for primary key

• Multivalued stereotype <<multivalued>>


Primary key is a unique identifier; no two Student instances will have the same studentId value.


Multivalued attribute can contain multiple values; a student may have several phone numbers


Composite attributes have multiple components.They are treated as separate classes in conceptual data models.The attribute address is defined to be of type Address.


What Is a Relationship?What Is a Relationship?

A semantic connection between objects of one or more classes

In UML, represented as a line connecting two or more class boxes (or connecting one class to itself)

In UML, known as an association


This is association between two classes, and is known as a binary association: an employee works in a department.Binary associations are the most common type.


We use an adornment to indicate how to read the association: “an employee works in a department”

works in


Roles identify the purpose of each class in the relationship.•Rolenames are useful (much) later on when we have software classes with attributes referring to instances of classes•Note that neither class has an attribute referring to object(s) of the other class – that would be an implementation detail


What Is Relationship Degree?What Is Relationship Degree?

The number of classes that participate in a relationship

Main degrees:

– Unary – a relationship between objects of the same class

– Binary – a relationship between objects of two different classes

– Ternary – a relationship between objects of three different classes

– !NOTE. Our text differs from other texts in these definitions:

• Degree is the number of objects that participate – there is no Unary relationship.

• Instead of unary, the term recursive is used

From text


A reflexive association

reflexive


This example is related to a business rule that states a specific quantity of a Part is shipped by a Vendor to a Warehouse.

The ternary relationship is expressed as in the following slide:


An n-ary association, n>2, is shown with a diamond symbol

Part

Vendor Warehouseships

The diamond is the symbol used for a n-ary association


An analyst will likely promote the association to a full class. This changes the model so it now has four classes and three binary associations.

Part

Vendor WarehouseshipsShipment


MultiplicityMultiplicity

Associations can be:

– One – to – one

– One – to – many

– Many – to – many


MultiplicityMultiplicity

The range of the number of objects in a class that can or must be associated with each object of another class in an association.

* * represents zero or more0..* same meaning as above1..* one or more1..5 one, two, three, four, or five0,1 zero or one1 exactly one3 exactly three

Ranges use ..

Enumeration uses ,


Multiplicity notation is: min..max


Roles, especially for reflexive associations, make it clear how an object participates in the association.Here, one object is the supervisor and the other is the subordinate.


A reflexive association

What are the multiplicities?


Associative ClassAssociative Class

An associative class is both a class and an association. It contains attributes and/or participates in relationships in its own right

Represented as a class connected to an association with a dotted line

Typically appears with a many-to-many association – there probably are attributes that describe the association


A Certificate represents a relationship between an employee and a course, and has an attribute pertaining to that relationship


In a many-to-many association, an object of one class can be associated with an object of the other class at most one time. This is true irrespective of whether we have an associative class or not:

Employee Course* *

Employee Course* *

Certificate

completes


Suppose an employee can take the same course more than once. We must promote the associative class to a full class:

Employee Course* *

Employee Course

* *Certificate

1 1

dateCompleted

CertificatedateCompleted


Reconsider marriedTo. Do we need an associative class? Do we need to promote an associative class to a full class?


Sometimes associative classes have their own relationships with other classes.


Types of AssociationsTypes of Associations

• Association – no object is subordinate to any other.

• Aggregation – one class represents the whole, and the other represents the part, but there is no concept of ownership

• Composition – one class represents the whole, and the other represents the part; the whole contains or owns the part; the part cannot exist without the whole.

• Generalization/specialization –an association between a general thing (the superclass or parent class) and a more specific kind of thing (the subclass or child class)


Composition

Aggregation


GeneralizationGeneralization

• a generalization is an association between a general thing (the superclass or parent class) and a more specific kind of thing (the subclass or child class)

• Inheritance – A subclass inherits all the attributes and operations of its superclass; an instance of the subclass contains all the same information (plus more) as an instance of the superclass

• An instance of a subclass is also an instance of the superclass


Generalization (Ch 26)

Multiple subclasses can be grouped to indicate they are relatedsubclasses

LibraryMember

StaffMember StudentMember

It then becomes meaningful to consider certain constraints:

complete, incomplete, disjoint, overlapping


Generalization

Inheritance of attributes and behaviour:•everything a LibraryMember can do, a StaffMember can do•everything a LibraryMember can do, a StudentMember can do

•If a LibraryMember can borrow a book, so can a StaffMember and a StudentMember

•a StaffMember and a StudentMember have all the attributes the LibraryMember has, and possibly moreSpecialization: there are some things that a specialized class can do that a LibraryMember cannot

LibraryMember

StaffMember StudentMember


example:

•a SavingsAccount is a specialized kind of BankAccount•a ChequingAccount is a specialized kind of BankAccount•a BankAccount is a specialized kind of Asset •RealEstate is …

Asset

RealEstate BankAccount Security

SavingsAccount ChequingAccount


PaymentAmount: money

Cash Payment Credit Payment Cheque Payment

SalePays-for1 1

Every payment, regardless of whether it is cash, credit, or cheque, has an Amount and it is associated with a Sale

CreditCard Cheque

1

1

1

*


Motivation for partitioning a conceptual class into subclasses:

•subclass has additional attributes of interest

•subclass has additional associations of interest

•subclass is operated on, handled, reacted to, or manipulated differently than the superclass or other subclasses

•the subclass concept represents an animate thing that behaves differently than the superclass or other subclasses


Using the UML symbol


Multiple inheritance is complicated and not supported by all object-oriented programming languages.


This is a difficult model to work with. An employee could be a Secretary paid hourly, but another Secretary may be paid a salary.


Consider:


Instead of Figure 7.23, a modeler might use:

Part

BasicPart Aggregate

1..*

What is this?

What does this allow us to build?

Are there some common attributes for BasicPart and Aggregate? Are there common operations?

Composed of

subpart


SummarySummary

We have covered:

– Developing data models from use cases

– UML notation for conceptual modeling

– Associations and multiplicity

– Aggregation, composition, and generalization

– Attributes: identifier, composite, multivalued, derived attributes

R McFadyen 7-192.2911 Chapter 7 Conceptual Data Modeling.

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Transcript of R McFadyen 7-192.2911 Chapter 7 Conceptual Data Modeling.