Introduction to Data Modelling

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Introduction to Data Modelling
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Data Modeling in IS Development

• Information System (IS)
• A system that processes data to produce
  information
• When analysing the existing system
• View of current data in the system (existing data
  view)
• Current processes that operate on the data and/or
  produce data (process view)
• Making sure these tally (event/transaction view)

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• When designing a new system

• A view of proposed data in the system (proposed
  data view)
• Required processes to operate on data and/or
  produce data (process view) to satisfy some user
  requirement
• Making sure these tally (event/transaction view)
• Note
• No mention of a computer system or
  computerisation!

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Entity-Relationship Modelling

• Top-down approach to data modelling
• Start by identifying the entities and their
  associations/relationships
• Draw up an ER Diagram (ERD)
• using appropriate notation
• Applying rules, translate ERD into a set of
  well-normalised tables

• The creative bit is the identification of
  appropriate entities and their relationships
• The rest is learning how to adhere to and apply
  appropriate rules of ER modelling correctly

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ER Modelling - How it works

• Entity
• thing
• independent existence
• uniquely identifiable
• something you need to keep a list of!
• E.g. employee, order, machine, property
• Nouns
• Defined in terms of their properties

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Attributes

• Attribute
• property of an entity
• e.g.
• an employees attributes might be
• national insurance number, salary, date of birth
• an orders attributes might be
• date of order, order number, customer ID
• a machines attributes might be
• model number, capacity, description
• a propertys attributes might be
• property address, number of bedrooms, price

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Relationships

• Relationship
• association of some kind between entities
• Verbs
• e.g.
• employee and machine
• relationship might be works on
• An employee works on exactly one machine
• order and stock
• relationship might be relates to
• An order relates to many stock items

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Identifying Entities and Attributes

• How to tell what is
• an appropriate entity?
• an appropriate attribute?
• Experience
• practice makes better
• Reading hints in scenario
• if scenario based - especially in exams!
• Acting on information from fact finding
  investigations
• Never expect to get it right the first time in
  the real world
• (or the second!)
• E-R modelling is an iterative process
• It is a CREATIVE process

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Reading the hints in the scenario or acting on
information from fact finding investigations

• Boundary of the problem
• e.g, client wants
• a computerised leisure centre booking system
• a computerised video rental system
• you to design a booking system for a village hall
• Entities actually highlighted explicitly
• record details of bookings, videos and
• Attributes actually highlighted even if entities
  arent
• the secretary keeps a record of the order
  number, date of order and what items of stock are
  held together with their respective quantity

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• Nouns and verbs
• sometimes provide a hint as to entities and their
  relationships
• can see them as Enterprise Rules
• e.g. a customer can make several orders, but an
  order relates to just one customer

• Look for Enterprise Rules either at attribute
  level or at entity level
• a part description can be the same for several
  part numbers, although part numbers are unique
• A part may be used in several machine repairs,
  and a machine repair may involve at least one
  part

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Entity Type vs Entity Occurrence

• Project vs P1, Machine Deployment System
• Employee vs E1, MR JG Browning
• Machine vs Lathe, 10110
• Entity-Relationship Modelling
• Entity(Type)-Relationship Modelling
• ButWe are interested in Entity(Occurrence)-Relat
  ionship(Occurrence) Diagrams to illustrate
  concepts (more later)

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Notations

• Many notations are used for Entity-Relationship
  modelling.
• The one I have used here (the Howe notation) is
  particularly good for beginners.
• Seeing examples of more than one notation is
  useful as so many exist.
• The concepts are the same whatever the notation.

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ER Diagram

• An ER Diagram is a pictorial representation of
  Entity (type) level enterprise rules within the
  boundary of what exists or is required
• Basic notation of ER Diagram
• NB - the colours are not part of the syntax

Always hard to name the relationship
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Degree of a Relationship11 Relationship (one to
one)

• Enterprise Rules Represented
• An employee works on at most one machine
• A machine is worked on by at most one employee

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Entity Relationship Occurrence Diagram 11

• Employee Works on Machine
• E1 M1
• E2 M2
• E3 M3
• E4 M4
• E5

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1M Relationship (one to many)

• Naming convention from 1 to M here
• Often we name relationships from M to 1

• Enterprise Rules Represented
• An employee may work on one or many machines
• A machine is worked on by at most one employee

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Entity Relationship Occurrence Diagram 1M

• Employee Works on Machine
• E1 M1
• E2 M2
• E3 M3
• E4 M4
• E5

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MN Relationship (many to many)

• Enterprise Rules Represented
• An employee may work on one or many machines
• A machine is worked on by one or many employees
• Equally you could use MM instead of MN

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Entity Relationship Occurrence Diagram MM

• Employee Works on Machine
• E1 M1
• E2 M2
• E3 M3
• E4 M4
• E5

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Complex Relationships

• Typically MNP for 3 entity relationships
  (MMM)
• MNPR for 4 entity relationships (very rare) or
  even more!!!!

• Enterprise Rules Represented
• A supplier may supply one or more parts to one
  or more customers
• A customer may be supplied with one on more
  parts from one or more suppliers
• A part may be supplied by one or more suppliers
  to one or more customers

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End of Lecture