Logical Data Modelling

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Logical Data Modelling
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Objectives

• To provide a definition of the terms Logical Data
  Model (LDM) and Logical Data Structure (LDS)
• To explain what a LDS consists of
• To explain a step by step process by which a LDS
  can be constructed

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What is a Logical Data Model?

• An abstract representation of the structural data
  requirements of part or all of an organisations
  operation

Organisational Activity
Abstraction

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What do Logical Data Structures Consist of ?

• Entity Types
• Relationship Types

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What is an Entity/Entity Type ?

• An Entity Type is a logical grouping of data
  relevant to the application in question
• An Entity is an Occurrence of an Entity Type
• An Entity Type would be represented in a RDBMS as
  a table.
• An Entity would be represented as a single row in
  a table

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Features of Entity Types

• An Entity Type can be a physical object, or an
  abstract concept or an event or anything else.
• It must be relevant.
• It must be identifiable.
• There must be the possibility of more than one
  occurrence.

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• Soft Rectangles
• Containing the name of the Entity Type

Person
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What is a Relationship Type ?

• A Relationship Type is a relevant business
  association between two Entity Types.
• A Relationship is an occurrence of a relationship
  type.

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• One to One (11)
• One to Many (1M)
• Many to Many (MM)

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Order Part
Order Part
Order Line
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Recursive Relationships
1M Recursive Relationships
Manages Managed By
Person
MM Recursive Relationships
Consists of Contributes to
Part
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Resolving MM Recursive Relationships
Consists of Contributes to
Part
Part Part Structure
Consists of Contributes to
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Mutually Exclusive Relationships

• Exclusive Ownership
• Exclusive Membership

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Exclusive Ownership
Organisation
Supplier Purchaser of Part of Part
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Exclusive Membership
Course Type Course Run Booking
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Types of 1M Relationships

• Owner Optional Member Mandatory
• Owner Mandatory Member Mandatory
• Owner Optional Member Optional
• Owner Mandatory Member Optional

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• A Customer may exist without placing any orders
• An Order must have been placed by one and only
  one Customer

Customer Order
place placed by
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Owner Mandatory Member Mandatory

• An Order must consist of at least one Order Line
• An Order Line must appear on one and only one
  Order

Order Order Line
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Owner Optional Member Optional

• A Employee may negotiate Orders if their job type
  is Salesperson
• An Order may be received directly from a customer
  and could thus exist independently of Employees

Employee Order
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Owner Mandatory Member Optional

• A Commission Plan will only exist if at least one
  Employee is being paid via that plan
• An Employee who is not a Salesperson doesnt get
  commission

Commission Plan Employee
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Entity/Event Modelling
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Objectives

• To explain the terms Entity/Event Model and
  Entity Life History
• To describe what ELHs consist of
• To introduce a step by step procedure for
  creating ELHs

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What is an Entity/Event Model?

• An Entity/Event Model is an abstract
  representation of how the entities in a system
  are affected by business events

Organizational Activity
Abstraction


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What do Entity Life Histories Consist of?

• Events
• Effects
• Structural Components

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What is an Event?

• A business event is a stimulus which causes a
  system to create, update or delete an occurrence
  of one or more entity types

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Types of Event

• External Events
• Internal Events
• Time Based Events

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What is an Effect?

• An effect is the change within a single entity,
  i.e. creation, update or deletion caused by an
  event
• Question- Do ELHs show events or effects?

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Structural Components

• Sequence
• Selection
• Iteration
• Parallelism

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Sequence

• A is a structural component which shows that
  effect B is followed by effect C

A
B C
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Selection

• A is a structural component which shows that
  either effect B takes place or effect C but
  not both

A
B O C O
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Iteration

• A is a structural Component which shows that
  effect B takes place 0 or more times

A B
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Parallelism

• A is a structural component which shows that
  either, both (in any order on any number of
  occasions) or neither of effects D and E may
  occur

A
B C D E
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Complete Example
Customer First Order Main Life Outstanding Pla
ced Cycle Balance 0 and No Orders
for 6 Mths Detail Changes Balance
Changes Detail Change Balance Change
Nam O Addr O Tel O Pymnt O Order
O Made Accptd