Ch 02 DataModelling I

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Business Information Systems

Data Modeling - I


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What is a Data Model?

student id

student last name

student first name

student major

STUDENTcourse id

course title

course number of credits

course instructor name

COURSE

attends/is taught to

DATA MODEL

The specification ofdata structures andbusiness rules to represent business

requirements.

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Entities

Each entity should be fully defined by the businesscommunityto: Identify why the business needs this information

Improve understanding

Avoid redundancy

Each ENTITY should be named using: Unique entity names in a model

A non-technical, business-like name

A singular noun that describes a singular instance (no collective

nouns)

ENTITYA person, place, thing, event, or concept about which thebusiness keeps data.

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Defining the Entity

Each ENTITY represents

a set/collection of likeindividual objects calledinstances.

Two types of entities:

Independent: depends onno other entity for its

identification

Dependent: depends on oneor more entities for itsidentification

LINE ITEM

ORDER

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Attributes

attributesemployee first name

employee last name

employee address

employee phone number

EMPLOYEE

ATTRIBUTE

A distinct characteristic of an ENTITY forwhich data is maintained.

ENTITY Name(above the box)

employee id

Attribute Names :

Uniqueattribute names in amodel (or entity)

A non-technical, business-

likename

Asingular nounthat

describes a singularinstance (no collectivenouns)

All lowercase (Erwinstandard), with spaces

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Attribute Types

Two types of attributes:

Key Non-key

consultant id

consultant first name

consultant last name

consultant specialization

consultant hourly rate

CONSULTANTKey Attribute(s)(above the line)

Non-KeyAttribute(s)

(below the line)

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How do we determine keys?

CANDIDATE KEY

Any attribute or group of attributes which serves touniquely identify each instance of an ENTITY.

BOOK BOOK BOOK

author first name

author last name

book title

book edition

book publisherbook year published

book isbn

book lc catalog number

author first name

author last name

book title

book edition


book isbn


author first name

author last name

book title

book edition


book isbn


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Primary Keys

PRIMARY KEY

An ATTRIBUTE or group of attributes thatuniquely identifies an instance of the entity.

book isbn

author first name

author last name

book title

book edition

book publisher

book year published


BOOK

The primary key is always placedabove the line in an Entity

Factors to consider: Should be efficient

Must not contain any nullparts

Values must remain static

Should be a data element inyour control

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Surrogate Keys

transaction id

account id

customer id

cash machine id

transaction date

CASH MACHINE TRANSACTION

Composite Key Surrogate Key

account id

customer id

cash machine id

transaction date

CASH MACHINE TRANSACTION

SURROGATE KEYA contrived, non-intelligent, single-attribute key used to

replace a long composite key.

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The story so far

An entity is something about which we want to store information

Entities are identified through the system analysis processAn entity has attributes which are also identified during systemanalysis process

Information about an entity is stored in values of the attributes

Key attributes identify an instance of an entity uniquely

If there are more than one key attribute they are called candidatekeys

Based on the application one of them is chosen as the primary key

If it is not possible to have a single attribute as the key attribute,

the primary key is created as a composite of 2 or more non key attributes or a single but new and artificial attribute called surrogate key is created

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Relationships

MOVIE

MOVIE COPY

is rented as

RELATIONSHIP

A logical link between two entities thatrepresents a business rule or constraint.

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Relationships

is ordered from/sends usPART SUPPLIER

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1:1 One to one mandatory; One student has one

address1:N One to many mandatory ; A customer havingmultiple addresses

1:1 One to one optional; An employee may or may

not have a spouse1:N One to many optional; One order having manyorder items

M:N many to many; Many parts being supplied by

many suppliers


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Relationship Cardinality

CUSTOMER ORDERplaces

BOOKSTUDENThas

PASSENGERAIRPLANE SEAT is ticketed for

TYRECARrequires

N

A customer places zero, one, or more orders. An order relatesto only one customer.

Each student has one or more books. A book is with one student

Each seat will seat zero or one passenger. One passenger can sit ononly one seat.

Each car has exactly N tyres

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Creating ER Diagram

Identify and list all the entities of the system

For each entity list the attributesIdentify the candidate keys and choose the primary keybased on the application

For each pair of entities, decide if there is a relationship

If there is a relationship, decide what kind

Use tools like Visio to draw the ER Diagram

Discuss the ER model with end users (walkthrough)

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Relationship Rules

1:1 mandatory; this means that both are parts of larger

entity; One entity is merged with the other to create anentity which has attributes of both entities

1: 1 optional; primary key of one entity is placed asattribute of the entity which is optional

1:n optional/ mandatory; primary key of singular entityis placed as an additional attribute in the entity that canhave multi occurrences

N:M; create new artificial entity which has primary key

of both original entities as attributes

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OMS

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Product Order CustomerN M MN

Product Order Customer

N M

MN

Order Item

Note: For clarity in understanding the type of relationship, the relationship names are not metioned in this diagram


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Foreign Keys

FOREIGN KEY (FK)

A primary key of a parent entity that iscontributed to a child entity across arelationship.

contains

order number (FK)line item sequence number

product codeline item quantitylineitem numberline item description

LINE ITEMORDER

order number

order dateorder salesrepresentative

order customer


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Referential Integrity : What & Why ?

What is the impact of:

Inserting, updating, or deleting a Parent Primary Key value?

Inserting, updating, or deleting a Child Foreign Key value?

None of these actions should break the relationshipfrom Child to Parent

Options can be specified as to how the DBMS shouldmanage these actions to maintain referential integrity

REFERENTIAL INTEGRITY (RI)Rules that determine what happens when a

Parent or Child instance is inserted, updated or

deleted.

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Exercise

Identify the entity, attributes and the key

Order Date

Customer Name

Customer AddressCustomer Credit Rating

Order Value

Item 1 Desc

Item 1 unit price

Item 1 quantity

Item 1 Deliver DateItem 2 Desc

Item 2 unit price

Item 2 quantity

Item 2 Deliver Date

ORDER


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Apply the ER diagram rules for the data model of a

library system

Supplier Parts ProductN M MN


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Next Class

Data Normalization

Documents

Ch 02 DataModelling I