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Database Systems

LECTURE #:09

M. Shafiq

FACULTY OF CS & IT

UNIVERSITY OF GUJRAT


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Data Modeling

A detailed model that captures overall structure of

organizational data while being independent of

any implementation considerations

Typically data modelling is carried out parallelwith other requirements analysis and structuring

steps during the systems analysis phase.

Following this, the data model supporting the

scope and requirements of the proposed system isdeveloped.

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Entity Relationship Diagram Model:

A Data Modeling Approach. Proposed by Peter Chen in 1976

Entity Relationship Diagram is a form of data modelling

A logical representation of the data of an organization or

business area in graphical form

Data modelling involves examining the data objects in a

system and identifying the relationship between these

objects

Top-down approach to database design

The primary purpose of an ERD is to document the logical

structure of a database

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Why ERD ?

Conceptual design: (ER Model is used atthis stage.)What are the entitiesand relationshipsin the

enterprise?

What information about these entities and relationshipsshould we store in the database?

What are the integri ty constraintsor business rulesthat hold?

A database `schema in the ER Model can berepresented pictorially (ER diagrams).

An ER diagram can be mapped into a relationalschema.

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Basic Symbols

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Components

Name

Symbol

Entity

Attribute

Relationship

Department

Staff No.

employsemploys

OR


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An Entity

An entity is a thing of interest to a systemabout which information is kept

For example in a Hospital AdministrationSystem, some likely entities would include:Patient, Doctor, Operation, Ward

Each of these things are of interest to thesystem and would have data stored aboutthem

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Choosing Entities

Example

A sale always starts with a customer

receiving an estimate. The estimate then

becomes an order. An order can be forone or more stock items. Each stock item

belongs to a certain stock category (e.g.

taps, sinks, cupboards etc.)

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An Attribute

An attribute is an item of data held about an

entity

In computer systems an attribute is a field

of information

Example : Sales System

Entity : Customer

Attributes : Customer Name, Customer

Address, Customer Phone Number

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For Example

In a Sales System, customer name and

address are attributesof the entity

customer.

The primary keyis likely to be Customer

Code or Customer ID as the unique

identifier for each customer in the system.

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Entity

symbols

Relationship

symbols

Attribute

symbols

A special

entity that isalso a

relationship

Basic E-R Notation


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Associative Entity

The database relational model doesn't offer direct supportto many-to-many relationships.

The solution to this problem is the creation of another table

to hold the necessary information for this relationship. This

new table is called an associative entity. An Associativeentity becomes an entity and a relationship since it

encapsulates properties from both.

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Multivalued Attribute

Multivalue attribute is the practice of maintaining more than a singlevalue in a database column.

If you want to know the hobbies for a client. The proper way to handle

this is to maintain the data in a one - to - many child table.

Another way is a multivalue field that will contain one or more values,

separated by a comma.

The eMailAddress field can store a variable number of email address

values in the single record.

So the list [[email protected],[email protected],

[email protected]] can be stored and accessed via a single

query / disk read when accessing the associated record.

phone number = {617-555-1234, 617-333-4321}


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Derived Attribute

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Derived attributes are attributes whose

values are generated from other attributes

using calculations, algorithms or procedures

For Example Employee Age derived from Date of Birth

Account Balance derived from ??

Work Experience derived from ??


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Entity with a multivalued attribute (Skill) and derived attribute

(Years_Employed)

Derivedfrom date employed and current date

Whats wrong with this?

Multivalued:an employee can have

more than one skill


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Composite Attribute

A composite attribute consists of a group of valuesfrom more than one domain. For example, the

Address attribute consists of several domains such

as house number, street number, city, country, etc.

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Strong and Weak Entity

A Weak Entity is an entity that cannot be uniquelyidentified by its attributes alone; therefore, it must use a

foreign key in conjunction with its attributes to create a

primary key. The foreign key is typically a primary key of

an entity it is related to. A weak entity is part of an ER diagram that relies on other

entities (Strong Entities). Without the strong entity, the

weak entity wouldn't exist.

An entity transaction has attributes transaction-number,date and amount. Different transactions on different

accounts could share the same number.

These are not sufficient to form a primary key

(uniquely identify a transaction).

Thus transaction is a weak entity set. 16


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Identifying Relationship

and Relationship An identifying relationship describes a

situation in which the existence of a row in

the child table depends on a row in the

parent table. An example of this that isgiven is,

An author can write many books (1-to-n

relationship), but a book cannot existwithout an author.

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Scenario Example of ERD

An Organization wants to run a homeappliances service center. Customers call to

request a technician and the center

Coordinator assigns an employee to sit forthe customer from a list of employees

available for the particular day requested.

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Service Center System

Data is used to:

Assign employee to job

Determine availability

Data to be captured in database:

Employee data

Job dataCustomer data

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ERD - Service Center System

Employee

Job

Customer

assigned to Related to


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Service Center System

Employee

Attributes: EmpID, Name, Address, Phone

Number, Available Hours, JobNo

Job Attributes: JobNo, JobDate, JobTime,

EmpID

Customer Attributes: CusNo, Name, Address, Phone

Number, JobNo

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Entity Instance

An entity instance is a single occurrence of anentity type.

An entity type is described just once in a data

model while many instances of that data typemay be represented by data stored in the

system.

There is one EMPLOYEE entity type inmost organisations but there may be

hundred or thousands of instances of this

entity type in the company.


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Example

A sale always starts with a customer

receiving an estimate.The estimate then

becomes an order. An order can be forone or more stock items. Each stock item

belongs to a certain stock category (e.g.

taps, sinks, cupboards etc.)


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From the narrative we can deduce

An Estimate becomes and Order

An Estimate is issued to a particular

Customer

An Estimate refers to a Stock Item(s)

An Order is placed by the Customer

An Order refers to a Stock Item(s)

A Stock Item refers to a particular Stock

Category


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Customer

Order

Stock Item

Stock

Category

Estimate

Places

Refers to

Becomes

Issued to

Refers to

Refers to

Order System


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Entity Primary Key Other

Attributes

Estimate Estimate Number Date

Customer Number

Stock Item Code

Order Order Number Same as Estimate

Customer Customer Number Customer Name

Customer Address

Credit Limit

Stock Item Stock Item Code DescriptionNumber in Stock

Supplier Code

Stock Category Category Code Category

Description


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ERD Relationships

A relationship is an association betweentwo entities that is important to the system.

We must also consider the extent to which

each entity is related to another. There maybe:

Unary Relationship

Binary Relationship

Ternary Relationship

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Cardinality defines the minimum and maximumnumber of occurrences of one entity for asingle occurrence of the other

From the previous example we can say that:

A customer can place several orders if they

wishBut an order can only be placed by one

customer

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The Cardinality of a Relationship


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Cardinality is bi-directional

This means that it must be defined for both

directions of the entity relationship.

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Cardinality


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A one-to-one relationship occurs where asingle occurrence of one entity relates to one

occurrence of a second entity.

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One-to-One Relationship (1:1)

Estimate Order

An estimate can only become one order and an

order is derived from one estimate.

becomes


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One-to-Many relationships occur when asingle entity instance is related to many

occurences of a second entity.

One-to-Many Relationship(1:N)

Customer Estimate

One customer can be issued with many estimates (the

relationship is described as one-to-many)

is issued

with


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Many-to-many relationships occur where manyinstances of an entity relate to many instances of a

second entity

Usually N:M relationships are broken into oneor more pairs of one-to-many relationships

Many-to-Many Relationship (N:M)


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Many estimates refers to many stock items (or many stock

items are referred to by many estimates)

N:M

Estimate Stock ItemRefers

to


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Sales ERD

Customer

Order

Stock Item

Stock

Category

Estimate

Places

Refers to

Becomes

Issued to

Refers to

Refers to


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Optional Cardinality Where the analyst considers whether an entity

occurrence at one end of the relationship can ever be

present in the system without the presence of the

corresonding occurrence of the entity at the otherend of the relationship

Mandatory Cardinality

When the occurrence of the entity at either end of

the relationship must be present in the system

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Cardinality


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Some relationships are mandatory and someare optional

Mandatory - Mandatory

Doctor PatientIs responsible

for

A doctor must be responsible for one or more patients

and a patient must be registered with one and only one

doctor

Registered with


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OptionalMandatory

Doctor PROJECTIs responsible

for

A doctor may be responsible for one or more patients

and a patiente must be registered with one and only

one doctor

Registered with

d i l


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A doctor must be responisble for one ofmore patients and a patient may be

registered with one and only one doctor

MandatoryOptional

Doctor Patient

Is

responsible

for Registered with

O i l O i l


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A doctor may be responsible for one ormore patients and a patient may be

registered with one and only one doctor

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OptionalOptional

Doctor Patient

Is

responsible

for Registered with


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Relationship CardinalityGraphic Cardinality

Exactly 1

0 or 1

1 or more

0 or many

More than 1

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Note

|| denotes 1 and only


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EE-R Data Model

Most common feature is representation of

supertypes and subtypes

A popular feature of Object Oriented paradigm


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Super/Subtypes

Also called generalization/ specialization

Supertype is called a General Entity type

whereas subtypes are the specializations

For Example

Wahical is Supertype and Car,Van and Bus arethe sub types.


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Super/Subtypes

PERSON

EMP FAC

ST

ST1 ST3ST2

General Entity Types

Specialized Entity Types


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Super/Subtypes

EMP

SALARIED HOURLY

Grade

AnalSal

NoOfHrs

HourlyRate


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Super/Subtypes

PERSON

STD FAC

StId

CGPA

Qual

GradeClsName

Name

Id

Address

Documents

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