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Bite sized training sessions:Data Modelling – Part 1 of 2
Data Model Diagrams
Feb 2011Prepared by Guy BeauchampGroup Projects & IT
Objectives
• To understand– What is a data model … and what it is
not!– Why do data modelling
• To be able to– Read a data model– Build a data model– Critically review a data model
What is a data model?• Specification of the data that is required in order for
– The solution to meet it’s objectives – Processes to be able to run
• A data model comprises:– A diagram showing the required data dependencies– A set of data definitions required for each attribute on the
diagram
• Also referred to as:– Logical Data Model (LDM)– Entity Model– Entity Relationship Diagram (ERD)– Data Dictionary– Object Model – Class Diagram– Data Structure– …etc!
What a data model is not
• A physical design for storing data
• A database design• Database table definitions• Object specification
Why do data modelling?Data & Process Requirements
DatabaseDesigns
Physical data Specs
System Specs
Manual Procedures
DesignedAs
DesignedAs
DesignedAs Designed
As
Manipulate
Manipulate
Solution Specification
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 that
relationship is (the cardinality of the relationship).Attribute The atomic pieces of information that we need to know about entities.
Entity
• A real world thing– E.g. Customer, Product
• An event between 2 or more entities– E.g. Sale
“A real world thing or an interaction between 2 or more real world things.”
‘Type’ Entities
• Categorises other entities
• Holds information that applies to sets of other entities
• Very common
• Typical relationship cardinality is mandatory 1:M
• Relationship names add little value
Customer Type Customerbe described by
describe
e.g.Student, Premium, Business, Retail
Relationships
There are dependency business rules between two entities – can be shown as:
“How entities depend on each other in terms of why the entities depend on each other (the relationship) and what that relationship is (the cardinality of the relationship).”
NB: there are other notations.
Relationship Names
Always name relationships in at least one direction (except for those involving Type entities)
Especially useful when there is more than one relationship between two entities:
Flight
Airport
Departs from Arrives at
Arrival point forDeparture point for
Flight
Airport
Reading relationships
One Customer (entity) may (cardinality) purchase (relationship name) one or more (cardinality) SALEs (entity)
One SALE must be purchased by one CUSTOMER
Tip – start with the word “One”, never “MANY”
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”
Primary Keys•A 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
Which attribute makes the best primary key?Why?
No (attribute) Name (attribute)
10 Fred Bloggs (instance)
67 Freda Jones (instance)
Customer (entity)
Purchased
Purchased via
Discussion:
What is the PK of Sale?
Do PKs need to be shown at the ‘FK’ end of a relationship?
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
Primary Keys are the navigation method for relationships
5 Data Modelling “No-No”s
1.No repeating attributes on entitiesE.g. On a Customer entity “1st child name”, “2nd child name”…
2.No attributes on entities that do not depend on the primary keyE.g. On Customer entity “order date”
3.No “Many to Many”s between entitiesE.g. Product ordered by many customers, a customer orders many products
4.No “one to one”s between entities (usually)E.g. Customer has one Membership Card and a Membership Card is for one Customer
5.No circular relationships between entities (usually)E.g. next slide
Circular relationships
Circular relationships
How do we fix this circular relationship?
Process for producing a data model diagram
•identify candidate entities•select a central candidate entity•define the primary key•work through the rest of the candidate entities
• consider whether it is in scope• define the primary key• define the relationship(s) between the candidate entity and all other
candidate entities on the diagram• create description entities as needed• fully express the cardinality of the relationship(s)• name the relationship(s) as needed
•validate everything with the user•review and refine.
Next step: Data definitions……covered in part 2 of this bite
sized training session
Minor ExerciseI own a florist’s shop called My Florist.
I want to start emailing reminders to customers when special occasions are due for which they have brought flowers in the past – for example a spouse’s birthday.
Let’s draw up a data model to support that process.
An answer…
Major Exercise• You are business analysts working for a company
called re-Evolution Coffee Houses Ltd• You have been given a piece of work – ref handouts• Produce a data model showing
– Entities– Primary Keys– Relationships
• Suggestion: follow the process for producing a data model diagram 4 slides previously
• The business users will be available for questions
• If you need to make an assumption about business requirements or anything else then document it
• Time allowed: 1 hour• Deliverable:
– Flip chart data model– Flip chart assumptions
• Be prepared to present your data model to the other team
• Don’t worry about completing the exercise• Do worry about the quality of what you get through
Major Exercise
…and finally
• Any questions?
• Further resources…
• Feedback
• Thank-you!