Part3 Database Analysis and Design Techniques Chapter 05- Entity-Relationship Modeling

Part3 Database Analysis and Design TechniquesChapter 05-Entity-Relationship Modeling

Lu WeiCollege of Software and MicroelectronicsNorthwestern Polytechnical University

Database Systems

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Outline

• Introduction• Entity and Entity Types• Relationship and Relationship Types• Attributes• Strong and Weak Entity Types• Entity Type Schema• Structural Constraints• Problems with ER Models• Enhanced Entity-Relationship Modeling

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Outline


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Introduction

• Once the requirements collection and analysis stage of the database application lifecycle is complete and we have documented the requirements for the database application, we are ready to begin the database design stage.

• One of the most difficult aspects of database design is the fact that designers, programmers,

and end-users trend to view data and its use in different ways.

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Introduction

• Unless we gain a common understanding that reflects how the enterprise operates, the design we produce will fail to meet the users’ requirements.

• To ensure that we get a precise understanding of the nature of the data and how it is used by the enterprise, a model is needed for communication

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Introduction

• Requirements for the model– Non-technical– Free of ambiguities ( 无二义 )

• The Entity-Relationship (ER) model is one such example.

• ER modeling is a top-down approach to database design.

• ER modeling is an important technique for any database designer to master.

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Introduction

• The diagrammatic notations for representing ER model– Unified Modeling Language (UML)– The Crow’s Feet Notation– The Chen Notation

• The ER model provides a semi-formal notation that allows designers to create a high-level conceptual schema.

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Outline


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Entity and Entity Types

• Entity – An entity is an object in the real world that is uniquely

identifiable and has independent existence. • Entity Type

– Entities having the same properties are grouped together to form an entity type .

– Note that the convention is to use singular form for an entity type name.

– Can be objects with a physical (or real) existence or conceptual existence.

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• Examples

Physical existenceStaffPropertyCustomerStudent Conceptual existenceViewingInspectionSC

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• Diagrammatic representation of entity types– Each entity type is shown as a rectangle labeled with t

he name of the entity, which is normally a singular noun.

– The first letter of each word in entity name is upper case.

Staff Branch

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Outline


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Relationship and Relationship Types

• Relationship – A relationship is a meaningful association between tw

o or more entities. • Relationship Type

– A set of meaningful associations among entity types.– A relationship type is a grouping of similar relationship

s.

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• About simantic net

Branch Has Staff

B003

B007

SG37

SG14

SA9

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Student Elect Course

s001

s003

c01

c02

c03

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• Diagrammatic representation of relationship types

Staff BranchHas

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• Degree of Relationship Type– The number of participating entity types in a

relationship.• The majority of the relationships in a database

are binary.

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• Diagrammatic representation of N-ary relationship types

Client

Registers

Staff

Branch

Staff registers a client at a branch

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Title

Group

Publisher

Collection

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Solicitor

ArrangersBuyer

Financial Institutio

nBid

A buyer( 买主 ), advised by a solicitor( 法律顾问 ) and supported by a financial institution( 金融机构 ), places a bid( 投标 ).

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• Questions – Is a ternary relationship is always equivalent and

hence can be replaced with three binary relationships?

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• Recursive Relationship– A relationship type where the same entity type particip

ates more than once in different roles.

Staff

Supervises

Supervisee

Supervisor Stude

nt

Supervises

Supervisee

Supervisor

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• Two entities may be associated through more than one relationship. Role may be used.

Staff BranchHas

ManagesManager Branch Office

Branch OfficeMember of Staff

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Outline


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Attributes

• Attribute – A property of an entity or a relationship type.

• Attribute domain– The set of allowable values for one or more attributes.

• Simple (atomic) Attribute– Attribute composed of a single component with an

independent existence.• Composite Attribute

– Attribute composed of multiple components, each with an independent existence.

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Attributes

• Single-valued Attribute– Attribute that holds a single value for each occurrence

of an entity type. • Multi-valued Attribute

– Attribute that holds multiple values for each occurrence of an entity type.

• Derived Attribute– Attribute that represents a value that is derivable from

value of a related attribute, or set of attributes, not necessarily in the same entity type.

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Attributes

• Candidate Key– Minimal set of attributes that uniquely identifies each

occurrence of an entity type. • Primary Key

– Candidate key selected to uniquely identify each occurrence of an entity type.

• Composite Key– A candidate key that consists of two or more

attributes.

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Attributes

• Diagrammatic representation of attributes

Staff

staffNo{PK}namepositionsalary/totalStaff

Branch

branchNo{PK}address street city postcodetelNo[1..3]

Has

Manages

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Attributes

• Attributes on Relationships

Newspaper

newspaperNamePropertyForRentpropertyNo

Advertises

dateAdvertcost

Newspaper advertises property for rent

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Outline


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Strong and Weak Entity Types

• Strong Entity Type– Entity type that is not existence-dependent on some

other entity type. • Weak Entity Type

– Entity type that is existence-dependent on some other entity type.

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Strong and Weak Entity Types

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Outline


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Entity Type Schema

• Entity Type Schema– An entity type schema specifies the common structure

shared by individual entities of that type. – Include a type name, name, and domain (value set) fo

r each entity attribute, and constraints on entities.

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Entity Type Schema

• Representation of entity type schema

MemNo: 5-digits long integersDriverLic: State 2-letter abbreviation and a 10-digit integer separated by a blankName: alphabetic string of length 30 containing blanks and hyphensAddress: character string of length 50PhoneNumber: character string of length 14

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Outline


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Structural Constraints

• There should be constraints placed on entity types that participate in a relationship.

• Do each branch must have staff?• The constraints on relationships are used to

express restrictions on relationships that exist in the real world.

Staff BranchHas

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• Main type of constraint on relationships is called multiplicity ( 多样性 ).

• Multiplicity– The number (or range) of possible occurrences of an

entity type that may relate to a single occurrence of an associated entity type through a particular relationship.

• Represents policies (called business rules) established by user or company.

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• In ER model, we can capture business rules using two types of multiplicity constraints, namely, cardinality ratio and participation.

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• Cardinality ratio ( 基数约束 )– Describes maximum number of possible relationship

occurrences for an entity participating in a given relationship type.

• Participation ( 参与约束 )– Determines whether all or only some entity

occurrences participate in a relationship.

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• The most common degree for relationships is binary.

• For cardinality constraint, binary relationships are generally referred to as being:– one-to-one (1:1)– one-to-many (1:*)– many-to-many (*:*)

• There are two types of participation constraints– Mandatory participation– Optional participation

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• Examples

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Staff

staffNoBranch

branchNoManages1 11.. 0..

One branch is managed by one member of staff

One member of staff manages one branch

Cardinality

Participation

All branches are managed (mandatory)

Not all staff manage branches (optional)

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• Examples

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Staff

staffNoPropertyForRentpropertyNo

Oversees0..1 0..*

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• Examples

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Newspaper

newspaperNamePropertyForRentpropertyNo

Advertises0..* 1..*

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• Multiplicity for Complex Relationships– Number (or range) of possible occurrences of an

entity type in an n-ary relationship when other (n-1) values are fixed.

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• Example

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Staff BranchRegisters

Client

1..1 1..1

0..*

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• Discuss about the multiplicity constraints between strong entity type and weak entity type.– A weak entity type must participate in a one-to-many r

elationship type with the identifying owner.– Weak entities have a total participation in the identifyi

ng owner relationship, and the identifying owner has a partial participation in the identifying owner relationship.

– Weak entities have a total participation in the identifying owner relationship, and the identifying owner has a partial participation in the identifying owner relationship.

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• Notation: – A total participation constraint does not imply that the

corresponding participating entity type is a weak entity.– The requirements of the system determine the choice

of strong entities versus weak entities versus multi-valued attributes.

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Outline


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Problems with ER Models

• Problems may arise when designing a conceptual data model called connection traps.

• Often due to a misinterpretation of the meaning of certain relationships.

• Two main types of connection traps are called fan traps and chasm traps.

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• Fan Trap– Where a model represents a relationship between

entity types, but pathway between certain entity occurrences is ambiguous.

• Chasm Trap– Where a model suggests the existence of a

relationship between entity types, but pathway does not exist between certain entity occurrences.

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• An Example of a Fan Trap

Staff BranchDivisionHas Operate

s1..11..11..* 1..*

How to know which members of staff work at a particular branch

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At which branch office does staff number SG37 work?

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• Restructuring ER model to remove Fan Trap

Staff DivisionBranchHas Operate

s1..*1..11..* 1..1

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SG37 works at branch B003.

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• An Example of a Chasm Trap

Branch PropertyForRentStaffHas Oversee

s0..11..*1..1 0..*

How to know which properties are available at each branch?

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At which branch office is property PA14 available?

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• Restructuring ER model to remove Chasm Trap

Branch PropertyForRentStaffHas Oversee

s0..11..*1..1 0..*

Offers

1..1 1..*

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• Discuss – When a fan trap may exist?– When a chasm trap may occur?

• A fan trap may exist where two or more 1:* relationships fan out from the same entity.

• A chasm trap may occur where there are one or more relationships with a minimum multiplicity of zero (that is optional participation) forming part of the pathway between related entities.

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Outline


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Enhanced Entity-Relationship (EER) Modeling

• Since 1980s there has been an increase in emergence of new database applications with more demanding requirements.

• Basic concepts of ER modeling are not sufficient to represent requirements of newer, more complex applications.

• Response is development of additional ‘semantic’ modeling concepts.

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• Semantic concepts are incorporated into the original ER model and called the Enhanced Entity-Relationship (EER) model.

• Examples of additional concept of EER model is called specialization / generalization.

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• Specialization / Generalization – Superclass

• An entity type that includes one or more distinct subgroupings of its occurrences.

– Subclass• A distinct subgrouping of occurrences of an entity type.

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• Specialization/generalization was introduced for two additional reasons:– To help in identifying the attributes and relationships specific to some entities, thereby enhancing the understanding of the application and adding more semantic clarity to the design.

– To avoid specifying attributes to an entity not applicable to it. This eliminates the need for a great number of NULL values.

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• Example

Staff BranchHas

ManagesManager Branch Office

Branch OfficeMember of Staff

Staff

Manager

Sales

secretary

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• The relationship between supclasses and its subclasses– Attribute Inheritance

• An entity in a subclass represents same ‘real world’ object as in superclass, and may possess subclass-specific attributes, as well as those associated with the superclass.

– Superclass/subclass relationship is one-to-one (1:1).– Not all members of a superclass need be a member

of a subclass.

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• The relationship between members of subclass– Superclass may contain overlapping or distinct

subclasses.

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• Constraints on Specialization / Generalization– Participation constraint

• Determines whether every member in superclass must participate as a member of a subclass.

• May be mandatory or optional.– Disjoint constraint

• Describes relationship between members of the subclasses and indicates whether member of a superclass can be a member of one, or more than one, subclass.

• May be disjoint or nondisjoint.

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• There are four categories of constraints of specialization and generalization:– mandatory and disjoint– optional and disjoint– mandatory and nondisjoint– optional and nondisjoint.

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• Diagrammatic representation of Specialization / Generalization

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• Specialization – Process of maximizing differences between members

of an entity by identifying their distinguishing characteristics.

• Generalization– Process of minimizing differences between entities by

identifying their common characteristics.

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• Aggregation

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• Composition

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Summary

• In this chapter you should have learned:– How to use Entity-Relationship (ER) modeling in a

database design– The basic concepts associated with the Entity-

Relationship (ER) model– A diagrammatic technique for displaying an ER model

using the Unified Modeling Language (UML) or the Crow’s Feet Notation.

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Summary

– How to identify and resolve some problems with ER models

– How to build an ER model from a requirements specification

– The useful additional data modeling concepts of Enhanced ER (EER) model.

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Part3 Database Analysis and Design Techniques Chapter 05- Entity-Relationship Modeling

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