CM515100-1 Business Systems Analysis, Design and Construction BSADC

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CM515100-1 Business Systems Analysis, Design
and ConstructionBSADC

• Week 6

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Relational Data Analysis 1 Entity Modelling a
top down approach
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Last Week

• Last week we looked at
• Low Level DFDs - DFD2
• Elementary Process Descriptions

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Data Flow Diagrams - DFDs

• The symbols - basic outlines

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Whats missing from the so far?

• DFDs identify and specify
• Data Flows
• Activities, Processes, Functions
• Data Stores
• They DONT identify and specify the structure of
  data stores (data tables)

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Objectives

• To discuss the 3 main concepts used in Entity
  Modelling and Data Analysis
• Establish those entities that are important to a
  particular system
• Understand the relationships between those
  entities
• Learn the associated structured modelling
  techniques the notation

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The Three main Concepts

• 1. The Entity
• (please dont confuse with external entities)
• 2. The Relationship (between entities)
• 3. The Attribute (belonging to an entity)

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An Elementary Example
Student Number Student Name Street
Address Town Post Code Telephone
Module Number Module Name Module
Leader Level Cats Points
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1. The Entity

• Is something we will need to hold data about
  within the system
• The context of the organisation determines
  whether an entity has significance or importance
  eg
• To a hospital
• Patient, Doctor, Clinic, Ward
• To a Delivery Company
• Driver, Vehicle, Consignor, Consignee,
  Consignment
• To a University
• Student, Lecturer, Lecture, Room, Module,
  Employee

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Drawing an entity
This is the symbol for the student entity
Student
Draw the symbol for the module entity
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2. The Relationship 1

• There are relationships between entities
• The relationship can be illustrated by a joining
  word or phrase
• Entity Entity
• Customer places Orders
• Hotel has Rooms
• Guest makes Bookings
• Student registers for Modules

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Adding a relationship
This is the notation for the relationship
Draw the relationship between student and module
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The Relationship 2

• Sometimes an entity can have a relationship with
  itself, for example,
• Entity Entity
• Person is married to Person
• Employee reports to Employee
• Product made from a Product

is made of
Product
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The notation an example so far
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The Notation a further example

• There are different types of relationships

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Degrees of relationship

• Relationships between entities are more detailed
  than first appears
• One to One (11)
• Customer has only one and only one account
• One to many (1N)
• Customer may have many bank accounts
• Hotel has many rooms
• Many to Many (MN)
• Project has many workers
• Worker works on many projects

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Modelling Degrees of relationship 1
One to One (11)
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Modelling Degrees of relationship 2
Many to Many (MN)
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Intersection (or Link entity)
This gives rise to ambiguity of file structure
A more practical model is
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Optionality

• A relationship between two entities can also be
  mandatory or optional
• Mandatory - a customer must have placed an
  order to be a customer
• - hotel must have a room
• Optionality - a bus may have a passenger
• - a hotel room may have a guest

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Modelling Optionality 1

• Mandatory

• Contingent

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Modelling Optionality 2

• Optional

Order item
Invoice item
may
may
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Exclusivity

• Some entities show links between two other
  entities - where the two links are mutually
  exclusive

Room
Bedroom
Bathroom
Living Room
Living Room
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Adding Attributes
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• Entities have attributes (data items)
• Attributes record something about an entity
• For example - The entity PERSON
• Surname First name Title Gender
• Smith John Mr M
• Jones Davy Mr M
• Marple Mindy Miss F
• Thatcher Margaret Ms F

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Attribute types and Attributes 1

• Be careful when you read the terms attribute type
  and attribute they are not the same thing
• Attribute types will translate into the fields of
  a record on a data table
• Attributes are the values or properties that
  identify or describe an entity occurrence

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Attribute types and attributes 2

• Entity Type Attribute Type Attribute
• Department Dept Name Accounts
• Building Canary Wharf
• Location Floor 3
• Dept Head Attila the Hun
• Cost Centre Code ACC3

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Getting started

• What are the data items associated with the
  following LDS diagram?

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The Student

• Entity Type Attribute Type Attribute
• Student Student Name Jones
• Street Address Leek Road
• Town Stoke-on-Trent
• Post Code ST4 2DE
• Telephone 294303

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The Module

• Entity Type Attribute Type Attribute
• Module Module Name SSAT
• Module Leader A Lecturer
• Level 1
• Cats Points 10

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The Data items identified
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Identifying occurrences of entities

• Each occurrence of an entity should be uniquely
  identified in some way
• Imagine the British Gas data base that used only
  surnames to identify account holders
• There would be 100,000 account holders called
  Jones in this country
• Even if we used the given names there would still
  be considerable duplication
• It would be impossible to find the right account
  by name alone

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Keys and Secondary keys

• Key
• a key is a data item used to identify a
  particular occurrence of an entity. It may not be
  unique so several entity occurrences may have the
  same key
• your surname is an example of a key
• Secondary Key
• a secondary key does not uniquely identify an
  entity but helps to further identify an entity
  occurrence
• your given name is a secondary key

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Primary and Compound Keys

• Primary Key
• a primary key is a key consisting of a data item
  that uniquely identifies an entity occurrence
• your national insurance number is unique
• as is your passport number
• a gas bill number is unique
• as is a car registration
• Compound keys
• this is a primary key that is made up of two data
  items
• room number and date booking reference
• also there are Foreign and Composite keys

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Foreign Keys

• Foreign Key
• An attribute in on table that is a primary key in
  another
• More later

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Adding a Primary Key
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Getting it right
Is this right?
The real situation is
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Putting it right
We need a link entity - less ambiguity
Student
Module
Student Number Student Name Street
Address Town Post Code ST4 2DE Telephone
Stud/Mod
Module Number Module Name Module
Leader Level Cats Points
Student Number Module number Date
enrolled Grade achieved
Foreign Keys
More about this soon
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Self Study

• Read Mastering Systems Analysis and Design -
  pages 67 to 78
• Do exercises 5.8.1 to 5.8.7
• Read handout

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Next week

• Data Optimisation

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Tutorial

• The college case study
• The library case study