ch04 er diagram 1

8/6/2019 Ch04 ER Diagram 1

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ITTelkom

Entity Relationship Diagram (1)

www.company.comwww.ittelkom.ac.id

CS2343 Perancangan Basisdata Relasional


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ITTelkom Outline

Modeling Entity, Relationship, Attributes

Mapping Cardinality Constraints Key

ER Diagram Design Issues


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ITTelkomModeling

A databasecan be modeled as: a collection of entities, relationship among entities.

An entity is an object that exists and is

s ngu s a e rom o er o ec s. Example: specific person, company, event, plant Entities have attributes

Example: people have namesand addresses

An entity set is a set of entities of the same type thatshare the same properties.

Example: set of all persons, companies, trees, holidays


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ITTelkomExample : Entityset Customer

Customer-Id Customer-name

Customer-Street

Customer-City

321-12-3123 Jones Main Harison

019-28-3746 Smith North Rye

Attribute

Entity 1

Entity 2

677-89-9011 Hayes Main Harison

555-55-5555 Jackson Dupont Woodside

CUSTOMER

Entity 4

Entity Set


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ITTelkomRelationship Sets

A relationship is an association among several entitiesExample:

Hayes depositor A-102

customerentityrelationship setaccountentity

A relationship set is a mathematical relation among n2 entities, each taken from entity sets

{(e1

, e2

, en

) | e1

E1

, e2

E2

, , en

En

}

where (e1, e2, , en) is a relationship

Example:

(Hayes, A-102) depositor


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ITTelkomRelationshipset Example - borrower

Customer-Id Customer-

name

Customer-

Street

Customer-City

321-12-3123 Jones Main Harison

019-28-3746 Smith North R e

Loan-

Number

Amount

L-17 1000

L-23 2000

677-89-9011 Hayes Main Harison

555-55-5555 Jackson Dupont Woodside

L-15 1500

L-14 500

L-19 900

CUSTOMER LOAN


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ITTelkomRelationship Sets

An attribute can also be property of a relationshipset.

For instance, the depositorrelationship set betweenentity sets customerand accountmay have theattribute access-date


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ITTelkomDegree of a Relationship Set

Refers to number of entity sets that participate in arelationship set.

Relationship sets that involve two entity sets arebinary (or degree two). Generally, most relationship

sets in a database system are binary. Relationship sets may involve more than two entity

sets.

Example: Suppose employees of a bank may have jobs(responsibilities) at multiple branches, with different jobs atdifferent branches. Then there is a ternary relationship setbetween entity sets employee, job, and branch


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ITTelkomExample : Degree of a Relationship Set

Binary Relationship

Student Teacher

Ternary Relationship

Employee Branch

Job


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ITTelkomAttributes Types

Simpleand compositeattributes.

Simple Attributes : attributes that cant be divided into differentparts ; Composite Attribute : attributes that can be divided into parts ;

example : Name, that can be divided into First Name and Last

Single-valuedand multi-valuedattributes Single-valued Attributes : attributes that only have 1 value Multi-valued Attributes : Attributes that can have many values in

the same domain ;

Example: multivalued attribute: phone_numbers, address

Derivedattributes Can be computed from other attributes

Example: age, given date_of_birth


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ITTelkomExample : Composite Attributes


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ITTelkomMapping Cardinality Constraints

Express the number of entities to which anotherentity can be associated via a relationship set.

Most useful in describing binary relationship sets.

cardinality must be one of the following types: One to one One to many

Many to one

Many to many


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ITTelkomMapping Cardinalities

One to one One to many

Note: Some elements in A and Bmay not be mapped to anyelements in the other set


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ITTelkomMapping Cardinalities

Many to one Many to many

Note: Some elements in A and B may not be mapped to any

elements in the other set


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ITTelkomKeys

A super key of an entity set is a set of one or moreattributes whose values uniquely determine each

entity.

A candidate ke of an entit set is a minimal su erkey Customer_idis candidate key of customer account_numberis candidate key of account

Although several candidate keys may exist, one of thecandidate keys is selected to be the primary key.


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ITTelkomKeys for Relationship Sets

The combination of primary keys of the participating entitysets forms a super key of a relationship set. (customer_id, account_number) is the super key of depositor NOTE: this means a pair of entity sets can have at most one

.

Example: if we wish to track all access_dates to each account by eachcustomer, we cannot assume a relationship for each access. We canuse a multivalued attribute though

Must consider the mapping cardinality of the relationship setwhen deciding what are the candidate keys


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ITTelkom

Keys Example

Customer = (Customer-Id, Customer-Name,

Customer-Street, Customer-City)

Super Key : - , - , - , -

Customer-Id, Customer-Name, Customer-Street Customer-Id, Customer-Name Customer-Id

Candidate Key :

Customer-Id

Primary Key : Customer-Id


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ITTelkomE-R Diagrams

o Rectangles represent entity sets.

o Diamonds represent relationship sets.

o Lines link attributes to entity sets and entity sets to relationship sets.o Ellipses represent attributes

l Double ellipses represent multivalued attributes.

l Dashed ellipses denote derived attributes.

o Underline indicates primary key attributes


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ITTelkomRelationship Sets with Attributes


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ITTelkomRoles

Entity sets of a relationship need not be distinct

The labels manager and worker are called roles ; they specifyhow employee entities interact via the works_for relationship set. Roles are indicated in E-R diagrams by labeling the lines that connect

diamonds to rectangles.

Role labels are optional, and are used to clarify semantics of therelationship


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ITTelkomCardinality Constraints

We express cardinality constraints by drawing either adirected line (), signifying one, or an undirectedline (), signifying many, between the relationship

One-to-one relationship: A customer is associated with at most one loan via the

relationship borrower

A loan is associated with at most one customer via borrower


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ITTelkomOne-To-One Relationship (1 1)

In the one-to-one relationship a loan is associatedwith only one customer via borrower, a customer isassociated with only one loans via borrower


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ITTelkomMany-To-One Relationships (N 1)

In a many-to-one relationship a loan is associatedwith several (including 0) customers via borrower, acustomer is associated with at most one loan via

borrower


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ITTelkomMany-To-Many Relationship (N M)

A customer is associated with several (possibly 0)loans via borrower

A loan is associated with several (possibly 0)customers via borrower


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ITTelkomParticipation of an Entity Set in a Relationship Set

o Total participation (indicated by double line) : every entity in

the entity set participates in at least one relationship in therelationship set

l E.g. participation of loan in borrower is total

o Partial participation : some entities may not participate in anyrelationship in the relationship set

l Example: participation of customer in borrower is partial


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ITTelkomDesign Issues

Use of entity sets vs. attributesChoice mainly depends on the structure of the enterprise beingmodeled, and on the semantics associated with the attribute inquestion.

Use of entit sets vs. relationshi setsPossible guideline is to designate a relationship set to describe anaction that occurs between entities

Binary versus n-ary relationship setsAlthough it is possible to replace any nonbinary (n-ary, for n> 2)

relationship set by a number of distinct binary relationship sets, a n-ary relationship set shows more clearly that several entitiesparticipate in a single relationship.

Placement of relationship attributes


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ITTelkom

Should addressbe an attribute of Employees or an

entity (connected to Employees by a relationship)? Depends upon the use we want to make of

address information, and the semantics of the

Entity vs. Attribute

a a:

If we have several addresses per employee, addressmust be an entity (since attributes cannot be set-valued).

If the structure (city, street, etc.) is important, e.g., wewant to retrieve employees in a given city, address

must be modeled as an entity (since attribute valuesare atomic).


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ITTelkomBinary Vs. Non-Binary Relationships

Some relationships that appear to be non-binary maybe better represented using binary relationships

E.g. A ternary relationship parents, relating a child to his/herfather and mother, is best replaced by two binaryrelationships, fatherand mother

Using two binary relationships allows partial information (e.g.only mother being know)

But there are some relationships that are naturally non-binary

Example: works_on


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ch04 er diagram 1

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