08 data modelling pt2

8/13/2019 08 Data Modelling Pt2

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Bite sized training sessions:Data ModellingPart 2 of 2

Data Definitions


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Objectives

To understand

What is a data model and what it

is not!Why do data modelling

To be able to

Read a data modelBuild a data model

Critically review a data model


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What is a data model? Specification of the data that isrequiredin order for

The solution to meet its objectives Processes to be able to run

A data model comprises: A diagram showing therequireddata dependencies

A set of data definitions required foreach attribute on thediagram

Also referred to as: Logical Data Model (LDM) Entity Model

Entity Relationship Diagram (ERD) Data Dictionary Object Model Class Diagram Data Structure

etc!


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What a data model is not

A physical design for storing

data

A database design

Database table definitions

Object specification


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Data Model Components

Entity A real world thing or an interaction between 2 or more real world things.

Relationship How and why entities depend on each other (the relationship) and what thatrelationship is (the cardinality of the relationship).

Attribute The atomic pieces of information that we need to know about entities.


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Attributes

No (attribute) Name (attribute)

10 Fred Bloggs ( instance)

67 Freda Jones ( instance)

Customer(entity)

Customer No Product No Date

10 101 21/2/2020

67 452 22/2/2020

Sale

No Name Price

101 Flange 123.00

452 Blitwort 34.50

Product

The atomic pieces of information that we need to know about entities


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Primary KeysA special kind of attribute, set of attributes and/or relationships

Is the way for the business to identify 1 unique instance of an entity

Certain rules apply to a primary key:

Must not be repeated within an entity

Once assigned can never be updated (only deleted)

Must be the way that the business uniquely identify an instance of an entity




Customer(entity)


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Primary Keys are the navigation method for

relationships

Purchases

Purchased via




Customer(entity)

Customer No Product No Date

10 101 21/2/2020

67 452 22/2/2020

Sale

No Name Price

101 Flange 123.00

452 Blitwort 34.50

Product


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5 Data Modelling No-Nos

1. No repeating attributes on entities

E.g. On a Customer entity 1stchild name, 2ndchild name

2. No attributes on entities that do not depend on the primary key

E.g. On Customer entity order date

The following should have been addressed via enti ty relationshipdiagram:

3. No Many to Manys between entities

4. No one to ones between entities (usually)

5. No circular relationships between entities (usually)


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A word about data definitions

There is no definitive set of information to define data

There is some basic information that will always beneeded

Do what you need to do in order to define the data

The following has worked during real-life project and is aproven starting point


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Process for defining the data

For each entity on the diagram

Define the entity

Define its relationships

Identifynon-key attributes from

Process specs

User input

Your analysis of requirements but always verify they are in scope!

Define attributes


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Data definitions - Entities

what the entity really islegal definition

Example: the Sale entity is a record of when a Product waspurchased by a Customer

The (non-functional)requirementsthe business has for entities

Attributes

example: Date, Quantity

Retention period

example: 6 yearsjustification: legal requirement

Max volume (max number of instances)

Example: 60,000 based on average number of sales per year of

10,000 x retention period Examples (not a requirement but very useful)

Example: On 28/4/2011 a flange was sold to Fred Bloggs


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Data definitions - Relationships

What the relationship really islegaldefinition

Example: For the relationship Customer

purchases Sale A Customer agrees to makeimmediate and full payment for a Product

which is given to the Customer at the time the

Sale is made.

The (non-functional)requirementsforrelationships is the cardinalityalreadydefined in the diagram


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Data definitions - Attributes What the attribute really islegal definition

Example: On entity Sale the attribute Date: the day and time theSale was agreed with the Customer

The (non-functional)requirementsthe business has for them

Data type

Example: Numeric

Size Example: 12

Domainnot all attributes will have a domain

Example: Timestamp YYYYMMDDHHMM

Rulesnot all attributes will have a rule(s)

Example: Cannot be a Sunday or Bank Holiday andmust be between 09:00 and 17:30 and

always display as YYYY-MM-DD:HH.MM

Examples (not a requirement but very useful)

Example: 2011-04-28:10.05


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Minor Exercise

I own a florists shop called MyFlorist.

I want to start emailing

reminders to customers whenspecial occasions are due forwhich they have broughtflowers in the pastforexample a spouses birthday.

We have already got a datamodel of the requirements solets define the data forCustomer.


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AnanswerEntity Definition Attributes Retention

Period

Max

Volume

Examples

Customer

Relationship DefinitionCustomer needs Reminder

Attribute Definition Data Type Size Domain Rules Example

Customer.Email

Address

Someone who

purchases flowers from

My Florist

Email Address 3 years 9900

(3 years, 302 working

days per year, 15

customers per day)

[email protected]

The customer has purchased flowers from My Florist for a Special Occasion in the past, so reminders need to be

sent just before the anniversary of the event.

The email address given by the

customer at the point of sale offlowers for a Special Occasion

Character Unlimited Email address None [email protected]


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Major Exercise You are business analysts working for a company called

re-Evolution Coffee Houses Ltd You have been given a piece of workref handouts

You have already produced a data model showing Entities

Primary Keys

Relationships

Define the data entities, relationships and attributes.

Suggestion: follow the process for producing a datamodel diagram 6 slides previously

The business users will be available for questions


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If you need to make an assumption aboutbusiness requirements or anything else thendocument it

Time allowed: 45 minutes Deliverable:

Flip chart data definitions

Flip chart assumptions Be prepared to present your data

definitions to the other team

Dontworry about completing the exercise

Major Exercise


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and finally

Any questions?

Further resources

Feedback

Thank-you!

08 data modelling pt2

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