richard merritt1 data modelling entities, attributes and relationships

1Richard Merritt

Data Data ModellingModelling

Entities,

Attributesand

Relationships

2Richard Merritt

Data ModellingData Modelling Technique for describing

information structures Information models represent:

things - entities properties of things - attributes associations between things -

relationships

3Richard Merritt

EntitiesEntities Abstractions of real world things

e.g. CUSTOMERdoes not relate to specific customers

...any distinguishable person, place, thing, event or concept about which information is kept. (Bruce 1992)

Specificcustomer

A

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AttributesAttributes The elements of data belonging to an

entity are known as its attributes

Customer

A/C No.NameAddressTel No.Credit limit

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RelationshipsRelationships Imagine two entities:

Lecturer and Student Lecturers teach students Teaching is the “relationship”

between the two abstract entities

6Richard Merritt

Logical Data ModelLogical Data Model

An Entity Type has a set of attributes

e.g. Customer has attributes ofAccount Number

Name Address

Telephone NumberCredit Limit

7Richard Merritt

Logical Data ModelLogical Data ModelAn Entity Type may have a number of occurrences.

Each Entity Occurrence has a unique set of values for the attributes.

CB

CustomerEntity Type

A/C No.NameAddressTel No.Credit limit

A

CustomerEntity Occurrences

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Logical Data ModelLogical Data Model

Customer EntityAttribute ValueAccount Number BL032

Name Bloggs & Son

Address 117 Acacia RdBirmingham 7

Telephone Number 0121 345678Credit Limit £2500

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A Table or RelationA Table or Relation

Reg. No. Surname Forename

45632

45315

44355

45422

Jones David

James Sarah

Jones Mary

Hill Simon

Each row of the table is unique.

10Richard Merritt

Entities, Tables & Entities, Tables & RelationsRelations

An Entity Type is represented as a Table (Relation)

Each Row (Tuple) of the Table is an Occurrence of the Entity

Each Column (Domain) of the Table contains the Values of one Attribute of the Entity

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Physical Data Physical Data OrganisationOrganisation

An Entity Type is usually implemented as a File in the Physical Storage Medium

Each Entity Occurrence is a Record in the File

The Value of an Attribute of the Entity Occurrence is stored in a Field within the Record

12Richard Merritt

Physical Physical OrganisationOrganisation

physical mediumfile

record

field

13Richard Merritt

one record

order no.customer

itemqty.

itemqty.

itemqty.

itemqty.

order no.customer

repeating fields

Repeating Attributes / Repeating Attributes / FieldsFields

Are repeating attributes (fields)

really attributes of this entity?

OrderFile / Entity

14Richard Merritt

Attributes / Fields of an Attributes / Fields of an OrderOrder

only one item

but …

One Entity Occurrence / Record

Order Customer Item QuantityNumber

1234 Ace XY345 8

Another Occurrence / Record

Order Customer Item 1 Quantity 1 Item 2 Quantity2...........Number

2156 Williams CCD3 2 TR805 5 ...........

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Entities or Entities or Attributes?Attributes?

When is data an Attribute of

an Entityand when is it a separate Entity?

Can one Entity ever be considered to be simply Attributes of another?

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Diagrammatic Diagrammatic RepresentationRepresentation

customer

Symbols for entities

hard box

soft box

student

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Diagrammatic Diagrammatic RepresentationRepresentation

order

Relationship between entities

item

crow’s footone

order can be

for many items

master

detail

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Degrees of Degrees of RelationshipRelationship

order

delivery

one to one

order

delivery

one to many

order

delivery

many to many

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Optional Optional RelationshipsRelationships

An Order must be for a

Customer

Customer

Sales Order

but

a Customer may not have any orders

optional at the customer end

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Consultant SystemConsultant System

A Client has an Account Consultants have a Grade and a

number of Skills Consultants are active on various

Projects Each Project is for one Account

21Richard Merritt

Possible Entities for Consultant Possible Entities for Consultant SystemSystem

Grade

Consultant

SkillProject

Account

Client

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Grade

Consultant SkillProject

Account

Client

Initial Attempt at Initial Attempt at RelationshipsRelationships

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Resolving One to One Resolving One to One RelationshipsRelationships

Client Account

The account is an attribute of the client and NOT a separate

entity.

Simply merge the entities which have a one to one relationship.

A Client can only have one Account.

24Richard Merritt

One to one relationships One to one relationships resolvedresolved

Grade


Client

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Resolving Many to Many Resolving Many to Many RelationshipsRelationships

Actor Scene

What is the nature of the relationship between an Actor

and a Scene?

Difficult to implement so how can they be replaced?

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Actor and Scene are both masters to the new linking

entity of Appearance.

Actor Scene

Appearance

Create a linking entity which is a detail to both the original

entities.

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What would make suitable entities and what attributes

might they have?

Patient Drug

?

Consultant Skill

?

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Many to many relationships Many to many relationships resolved resolved

Grade

Consultant Skill


Activity

ClientAvoid crossed relationshipsResuscitate dead crows!

29Richard Merritt

Developing a Developing a Logical Data Logical Data

StructureStructure identify possible entities

draw initial entity relationship diagram

resolve 1:1 and many:many relationships & check for further entities and relationships

remove redundant relationships

show optionality

30Richard Merritt

Cross-checking the Cross-checking the LDSLDS

The LDS is derived using a “Top Down” approach.

It can be cross-checked by using a “Bottom Up” approach, building up the entities from their

attributes. This technique is called Normalisation

which is the subject of the next lecture.

31Richard Merritt

Relational Data Relational Data AnalysisAnalysis

Normalisation

With thanks to Codd & Date.

32Richard Merritt

Attributes / Fields of an Attributes / Fields of an OrderOrder

only one item

but …

One Entity Occurrence / Record

Order Customer Item QuantityNumber

1234 Ace XY345 8

Another Occurrence / Record

Order Customer Item 1 Quantity 1 Item 2 Quantity2...........Number

2156 Williams CCD3 2 TR805 5 ...........

33Richard Merritt

NormalisationNormalisationobjectivesobjectives

to reduce data redundancy

to hold each data item (attribute) with as few occurrences as possible

to identify and remove any dependencies between data items stored together (in the same table)

34Richard Merritt

A RelationA RelationA two-dimensional A two-dimensional

tabletable

row (tuple)

Reg. No. Surname Forename4563245315

4435545422

Jones DavidJames SarahJones MaryHill Simon

column (attribute)

unique primary key

35Richard Merritt

A Compound A Compound KeyKey

Consultant No. Project No. Time (days)

001001003003

C232C979A176C232

3.010.55.0

20.0

36Richard Merritt

Attributes Attributes Listed Listed

Consultant No.

Project No.

Time (days)

Reg. No.

Surname

Forename

key attribute underlined

compound key

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Un-Normalised Un-Normalised DataData

Consultant DetailsNo. Name Address Grade Salary Scale

004 Mary Wheeler 236 Fore Street D S2

Ivybridge

DevonSkills

Code Description Qualification

SK01 Accounting IMA

SK10 CAD / CAM 3 yrs. AutoCAD

SK15 SSADM V4 Certificate

38Richard Merritt

AssumptionsAssumptions Consultant No., Skill Code and Grade are

unique. A consultant can have many skills each

identified by a Skill Code. For each skill only the consultant’s highest

Qualification is recorded. Other consultants may have the same skills

(and Skill Code) but not necessarily the same Qualification.

Each Skill Code has one Description. Each Grade belongs to one Salary Scale.

39Richard Merritt

Un-Normalised Form Un-Normalised Form (UNF)(UNF)

Consultant No.

Name

Address

Grade

Salary Scale

Skill Code

Description

Qualification

key should uniquely identify a row in the table

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UNFUNF

list all data attributes allocate primary key identify repeating

group(s)(optional)

UNF

Consultant No.NameAddressGradeSalary Scale (Skill Code Description Qualification)

41Richard Merritt

First Normal Form (1 First Normal Form (1 NF)NF)RuleRule

remove repeating data

Consultant No.

Name

Address

Grade

Salary Scale

Skill Code

Description

Qualification

42Richard Merritt

First Normal Form (1 First Normal Form (1 NF)NF)

Consultant No.

Name

Address

Grade

Salary Scale

Consultant No.

Skill Code

Description

Qualification

The compound key of the new table which contains the repeating group consists of the original key plus the attribute(s) which uniquely identify a single set of repeating values given a single value of the original key.

43Richard Merritt

1NF1NF separate repeating

group copy non-repeating

group unchanged add initial primary

key to repeating group & identify compound key

UNF


1NF

Consultant No.NameAddressGradeSalary Scale

Consultant No.Skill CodeDescriptionQualification

44Richard Merritt

Second Normal Form (2 Second Normal Form (2 NF)NF)RuleRule

remove part-key dependencies

Consultant No.

Skill Code

DescriptionQualification

45Richard Merritt

Second Normal Form Second Normal Form (2 NF)(2 NF)

Consultant No.

Name

Address

Grade

Salary Scale

Consultant No.

Skill Code

Qualification

Skill Code

Description

46Richard Merritt

2NF2NF

separate part-key dependencies

all other groups are copied across unchanged

do not omit key only groups

1NF



2NF


Consultant No.Skill CodeQualification

Skill CodeDescription

47Richard Merritt

Third Normal Form (3 Third Normal Form (3 NF)NF)RuleRule

remove inter-data (and inter-key) dependenciesConsultant No.

Name

Address

Grade

Salary Scale

48Richard Merritt

Inter-Data Inter-Data DependencyDependency

Salary Scale Grade

S1

S2

S3

ABCDEFG

49Richard Merritt

Third Normal Form (3 Third Normal Form (3 NF)NF)

Consultant No.

Name

Address

Grade

Grade

Salary Scale

Consultant No.

Skill Code

Qualification

Skill Code

Description

*

* denotes a Foreign Key

50Richard Merritt

3NF3NF separate inter-data

(non-key) dependencies

identify foreign keys

2NF




3NF

Consultant No.NameAddressGrade

GradeSalary Scale



*

51Richard Merritt

Normalisation of Consultant Normalisation of Consultant DataData

1NF



2NF




3NF


GradeSalary Scale



*

UNF


52Richard Merritt

NormalisationNormalisation StagesStages

UNF - list attributes & allocate primary key

1NF - remove repeating groups

2NF - remove part key dependencies

3NF - remove inter-data dependencies

53Richard Merritt

Third Normal Form Third Normal Form TestsTests

Given a value for the key of a 3NF relation, is there only one possible occurrence of the associated data (row)?

Is each attribute (column) of the relation dependent on the key, the whole key and nothing but the key?

54Richard Merritt


Grade

Salary Scale

*

CONSULTANT

GRADE

Consultant No.

Skill Code

Qualification

Skill Code

Description

CONSULTANT SKILL

SKILL

Relation (Table) Relation (Table) NamesNames

55Richard Merritt

Summary of the Summary of the NormalisationNormalisation

UNF 1NF 2NF 3NF

CONSULTANT


CONSULTANT SKILL


CONSULTANT

Consultant No.NameAddressGradeSalary ScaleSkill CodeDescriptionQualification

CONSULTANT


CONSULTANT SKILL


SKILL


CONSULTANT


GRADE

GradeSalary Scale

CONSULTANT SKILL


SKILL


*

56Richard Merritt

3NF Entity-Relationship 3NF Entity-Relationship DiagramDiagram

3NF Relations are Entities

Entities / Relations are linked by their common attributes

Relational Model

57Richard Merritt

Relationships from Relationships from 3NF3NF

Entities are linked by their keysKeys and Foreign Keys are the common attributes.

An Entity / Relation with a Compound Key is a DetailIts Masters are Relations that have as their Key a part of the

Compound Key

An Entity / Relation with a Foreign Key is a DetailIts Master is the Relation which has the Foreign Key as its whole

Key

58Richard Merritt

3NF Relations are 3NF Relations are EntitiesEntities

Consultant Skill

Grade

Consultant Skill

59Richard Merritt

Consultant Skill

Grade

Consultant Skill

Compound Keys are Compound Keys are DetailsDetails

master Skill Code

masterConsultant No.

detail withcompound key

Consultant No.

Skill Code

60Richard Merritt

Foreign Keys are Foreign Keys are DetailsDetails

Consultant Skill

Grade

Consultant Skill

masterGrade

detail with

foreign key

* Grade

61Richard Merritt

Cross-checking the Cross-checking the LDSLDS

The LDS is derived using a “Top Down” approach.

Normalisation is used to cross-check the LDS by using a “Bottom Up” approach, building up the entities and relationships from their attributes.

62Richard Merritt

Compare to LDSCompare to LDSGrade

Consultant Skill


Activity

Client

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Why are the diagrams not the same?What do they have in common?What are the differences? Why?

What do we need to resolve the

differences?

Normalise other documents / data sources.

LDS versus 3NF LDS versus 3NF Entity Relationship Entity Relationship

DiagramDiagram

64Richard Merritt

What is the What is the key?key?

Activity No

12121

Project No

C232C232C979C979A176

Time (days)

0.11.50.32.01.0

Activity Type

write letterdraw DFDinterviewplan projectvisit site

Activity No. is only unique within one project.

Consultant No

003001004004002

65Richard Merritt

A Composite A Composite KeyKey

Project No.

Activity No.

Consultant No.

Activity Type

Time (days)

composite key

a key of more than one attribute where part of the key does not have a unique meaning

66Richard Merritt

Data ModellingData Modelling Derive an LDS - “top down” Normalise all relevant data (documents etc.)

apply 3NF tests Merge relations with the same key

check for synonyms and homonymsapply 3NF tests again

Give each Relation (Entity) a namenames should be consistent with LDS

Draw a 3NF Entity Relationship Diagram Compare LDS & 3NF ERD and resolve

differences

67Richard Merritt

Another Another NormalisationNormalisation

A sample purchase order document.

Assumptions

Identify data items and work out rules and assumptions. It is critical to document the rules or assumptions of the data prior to normalisation. Assumptions may change over the life of the system.

Supplier No., Purchase Order No. and Part No. are unique.Cost Each depends only on Part No.

Supplier No. 54Name Wizcorp Ltd.

Purchase Order No. 46Order Date 15/2/99

Description Cost each Quantity

stainless bolt 10mm x 4cmstainless nut 10mm

3m x 8cm stainless round

0.500.20

82.50

100100

5

Part No.345763457934121

68Richard Merritt

Normalisation of Purchase Normalisation of Purchase

OrderOrder 1NF

Purchase Order No.DateSupplier No.Name

Purchase Order No.Part No.DescriptionCost eachQuantity

2NF

Purchase Order No.DateSupplier No.Name

Purchase Order No.Part No.Quantity

Part No.DescriptionCost each

UNF

Purchase Order No.DateSupplier No.Name (Part No. Description Cost each Quantity)

3NF

*

Purchase Order No.DateSupplier No.

Supplier No.Name



69Richard Merritt

Purchase Order Purchase Order Document 3NFDocument 3NF

*

Purchase Order No.DateSupplier No.

Supplier No.Name



Purchase Order

Supplier

Purchase Order Item

Part

apply 3NF tests

name entities (relations)

70Richard Merritt

Assumptions

Customer No, Order No and Part No are unique but Item is only unique within one order. Price each depends on Part No.. The same Part No can be used for more then one Item on the same order (but the Date Required will vary).

Sales OrderSales OrderCustomer No 54

Name J. Thomas

Order No 76

Date 5/2/99

Description

widget

assembly

rod

Price each

1.50

3.20

1.20

Part No

41235

23467

89965

Value

6.00

12.80

7.20

Item

1

2

3

Order Total 58.00

Date Required

4 23467 assembly 3.20

Quantity

4

4

6

10

32.00

19/2/99

19/2/99

19/2/9926/3/99

71Richard Merritt

Normalisation of Sales Normalisation of Sales

OrderOrder UNF

Sales Order No.DateCustomer No.NameOrder Total (Item Part No. Description Price each Quantity Value Date Required)

1NF

Sales Order No.DateCustomer No.NameOrder Total

Sales Order No.ItemPart No.DescriptionPrice eachQuantityValueDate Required

( )

2NF

Sales Order No.DateCustomer No.NameOrder Total

Sales Order No.ItemPart No.DescriptionPrice eachQuantityValueDate Required

)(

3NF

*

Sales Order No.DateCustomer No.Order Total

Customer No.Name

Sales Order No.ItemPart No.QuantityValueDate Required

Part No.DescriptionPrice each

( )*

72Richard Merritt

Sales Order Document Sales Order Document 3NF3NF

Customer No.Name

Part No.DescriptionPrice each

Customer

Part

Sales Order

Sales Order Item

*

Sales Order No.DateCustomer No.Order Total


*

( )

73Richard Merritt

Merged 3NFsMerged 3NFsPURCHASE ORDER

Purchase Order No.Purchase Order DateSupplier No.

SUPPLIER

Supplier No.Supplier Name

PURCHASE ORDER ITEM


PARTPart No.DescriptionCost eachPrice each

*

SALES ORDER

Sales Order No.Sales Order DateCustomer No.Order Total

CUSTOMER

Customer No.Customer Name

SALES ORDER ITEM


*

*

( )

74Richard Merritt

3NF Relations are 3NF Relations are EntitiesEntities

Sales Order Part

Customer

Sales OrderItem

Supplier

Purchase OrderItem

Purchase Order

75Richard Merritt

Sales Order Part

Customer

Sales OrderItem

Supplier

Purchase OrderItem

Purchase Order

Compound Keys are Compound Keys are DetailsDetails

masterPurchase Order No.

masterPart No.

detail withcompound keyPurchase Order No.Part No.

composite keySales Order No.Item

not compound

( )

76Richard Merritt

Sales Order Part

Customer

Sales OrderItem

Supplier

Purchase OrderItem

Purchase Order

Foreign Keys are Foreign Keys are DetailsDetails

Sales Order No.is foreign key

77Richard Merritt

LDS from Tutorial LDS from Tutorial ExampleExample

Stock Item

Supplier

Purchase Order

W arehouseSection Customer

Sales Order

DespatchPurchase OrderItem

Sales OrderItem

Despatch Item

78Richard Merritt

LDS versus 3NF LDS versus 3NF Entity Relationship Entity Relationship

DiagramDiagram

What are the differences?

What do we need to resolve these?

Normalise a despatch document.

richard merritt1 data modelling entities, attributes and relationships

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