Chapter 12/13 - Objectives CSS 225 Database Systems...

CSS 225 Database Systems

Chapter 12 Entity-Relationship Modeling

and Chapter 13 Enhanced E-R Modeling

Asst. Prof. Chukiat Worasucheep

Applied Computer Science

SCL 801 #9

KMUTT

Chapter 12/13 - Objectives

• How to use Entity–Relationship (ER) modeling in database design.

• Basic concepts associated with ER model.

• Diagrammatic technique for displaying ER model using Unified Modeling

Language (UML).

• How to identify and resolve problems with ER models called connection

traps.

• How to build an ER model from a requirements specification.

• Understand Enhanced ER (EER) model is called specialization and

generalization.

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Contents

• Basic concepts associated with ER model

▫ Entities and relationship types

▫ Attributes and keys

• Structural constraints

• Problems with ER models

• Enhanced ER (EER) model: Specialization and Generalization.

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@2012 by Chukiat Worasucheep

ER diagram of Branch user views of DreamHome

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Concepts of the ER Model

• Entity types

• Relationship types

• Attributes

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Entity Type

• Entity type

▫ Group of objects with same properties, identified by enterprise as having an independent existence.

• Entity occurrence

▫ Uniquely identifiable object of an entity type.

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Examples of Entity Types

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ER diagram of Staff and Branch entity types

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Staff Branch

Relationship Types

• Relationship type

▫ Set of meaningful associations among entity types.

• Relationship occurrence

▫ Uniquely identifiable association, which includes one occurrence from each participating entity type.

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Semantic net of Has relationship type

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ER diagram of Branch Has Staff relationship

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Relationship names

Relationship Types

• Degree of a Relationship

▫ Number of participating entity types in the relationship.

• Relationship of degree :

▫ two is binary

▫ three is ternary

▫ four is quaternary.

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Binary relationship called POwns

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Ternary relationship called Registers

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Quaternary relationship called Arranges

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Relationship Types

• Recursive Relationship

▫ Relationship type where same entity type participates more than once in different roles.

• Relationships may be given role names to indicate purpose that each participating entity type plays in a relationship.

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Recursive relationship called Supervises with

role names

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Entities associated through two distinct

relationships with role names

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Contents







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Attributes

• Attribute

▫ Property of an entity or a relationship type.

• Attribute Domain

▫ Set of allowable values for one or more attributes.

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Attribute Types: simple vs. composite

• Simple 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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Attribute Types: single vs multiple valued

• 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.

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Attributes

• 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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Keys

• 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. This should be clearly denoted in ERD.

• Composite Key

▫ A candidate key that consists of two or more attributes.

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ER diagram of Staff and Branch entities and

their attributes

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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.

• Note also that a relationship may also have attributes!

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Strong entity type called Client and weak entity

type called Preference

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Relationship called Advertises with attributes

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Contents







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Structural Constraints

• Main type of constraint on relationships is called multiplicity.

▫ Multiplicity refers to 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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• The most common degree for relationships is binary.

• Binary relationships are generally referred to as being:

▫ one-to-one (1:1)

▫ one-to-many (1:*)

▫ many-to-many (*:*)

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Semantic net of Staff Manages Branch

relationship type

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Multiplicity of Staff Manages Branch (1:1)

relationship

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Semantic net of Staff Oversees PropertyForRent

relationship type

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Multiplicity of Staff Oversees PropertyForRent

(1:*) relationship type

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Semantic net of Newspaper Advertises

PropertyForRent relationship type

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Multiplicity of Newspaper Advertises

PropertyForRent (*:*) relationship

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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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Semantic net of ternary Registers relationship

with values for Staff and Branch entities fixed

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Multiplicity of ternary Registers relationship

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Summary of multiplicity constraints

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• Multiplicity is made up of two types of restrictions on relationships: cardinality and participation. � P : C

▫ Cardinality

� Describes maximum number of possible relationship occurrences for an entity participating in a given relationship type.

▫ Participation

� Determines whether all or only some entity occurrences participate in a relationship. Basically there are two participation types:

� Mandatory (or all)

� Optional (or some)

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Multiplicity as cardinality and participation

constraints

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Contents







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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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Problems with ER Models

• 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

• It might occur when 1:m relationships fan out from a single entity.

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Semantic Net of ER Model with Fan Trap

• At which branch office does staff number SG37 work?

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Restructuring ER model to remove Fan Trap

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Semantic Net of Restructured ER Model with

Fan Trap Removed

• SG37 works at branch B003.

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An Example of a Chasm Trap

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• It might occur where there is a relationship with partial participation, which forms part of the pathway between entities that are related.

• The partial participation of Staff and PropertyForRent in the oversees relation means that some properties cannot be associated with a branch office through a member of staff.

Semantic Net of ER Model with Chasm Trap

• At which branch office is property PA14 available?

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ER Model restructured to remove Chasm Trap

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• We need to add the missing relationship which is called `offers' between the Branch and the PropertyForRent entities.

• Be careful when you remove relationships which you consider to be redundant.

Semantic Net of Restructured ER Model with

Chasm Trap Removed

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Contents







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Enhanced Entity-Relationship Model

• Since 1980s there has been an increase in emergence of new

database applications with more demanding requirements.

• Semantic modelling 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 subgroupingsof its occurrences.

• Subclass

▫ A distinct subgrouping of occurrences of an entity type.

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• Superclass/subclass relationship is one-to-one (1:1).

• Superclass may contain overlapping or distinct subclasses.

• Not all members of a superclass need be a member of a subclass.

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• 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.

• 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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AllStaff relation holding details of all staff

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Specialization/generalization of Staff entity

into subclasses representing job roles

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Specialization/generalization of Staff entity

into job roles and contracts of employment

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EER diagram with shared subclass and subclass

with its own subclass

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Constraints on Specialization / Generalization

• Two constraints that may apply to a 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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Constraints on Specialization / Generalization

• 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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mandatory optional

disjoint

nondisjoint

DreamHome worked example - Staff Superclass

with Supervisor and Manager subclasses

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DreamHome worked example - Owner Superclass with

PrivateOwner and BusinessOwner subclasses

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DreamHome worked example - Person superclass with

Staff, PrivateOwner, and Client subclasses

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Contents







• Exercises

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Exercises

• บรษิัทรถโดยสารเตา่ทองมรีถโดยสาร (Bus) หลายคัน แต่ละคันจัดไวส้ําหรับเสน้ทาง (Route) เฉพาะ แมว้่าบางเสน้ทางอาจจะมมีากกว่า 1 คัน แตล่ะเสน้ทางวิ-งผ่านเมอืง (Town) ตา่งๆ คนขับรถ (Drivers) 1 หรอือาจมากกวา่นั9นถกูจัดให ้ประจําแต่ละชว่ง (Stage) ของเสน้ทาง บางเมอืงมอีูร่ถ (Garage) สําหรับเก็บรถโดยสาร รถโดยสารแตล่ะคันมเีลขทะเบยีน (Registration number) และบรรทกุผูโ้ดยสารไดจ้ํานวนแตกต่างกันเนื-องจากมขีนาดแตกต่างกัน โดยบางคันจะมสีองชั 9น (ตามที-พบเห็นบอ่ยในอังกฤษหรอืฮอ่งกง) (Single or double-decked) แต่ละเสน้ทางจะมหีมายเลข (Route number) และมจีํานวนผูโ้ดยสารเฉลี-ยที-ใชใ้นแตล่ะวัน คนขับรถจะมหีมายเลขพนักงาน (Employee number) ชื-อ ที-อยู่ และอาจมีหมายเลขโทรศัพท์

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