ims 5024 data modelling (1). ims 5024 lecture 32 content individual assignment date pitfall...
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IMS 5024
Data Modelling (1)
IMS 5024 Lecture 3 2
Content
Individual assignment date Pitfall revisited Group assignment Class assignment Nature of data modelling Tools/Techniques used in data modelling Place in ISD Evaluation of data modelling Reading list
Date for the individual assignment Two dates appear in the hand out:
– 23 August 2001– 27 August 2001 (it also wrongly states that
it is a Friday)
What will it be!!!
–27 August 2001 at 17h00!!
Class assignment
What is this about?
What conventions/rules are used?
What does it depict?
Where would this fit into ISD?
Data modelling describe:
Structure Meaning Relationship
Of Data
Data modelling help us to grasp:
Static Data in the organisationFundamental building block of
the systemTwo perspectives (Process and
Data)
Techniques used in Data modelling Entity relationship diagrams Normalisation Data Dictionary
What difference? Use both?
Entity
Entity – things of interest to the business
Identification of an entity is subjective Entities can be:
• Real eg product• Abstract eg Quota• Event remembered eg sale• Role played eg employee
Employee
Relationship
Relationship Between entities
Cardinality (eg. One to many, one to one ect.)
Degree of relationship (Unary, Binary, Ternary)
DepartmentEmployee
Examples of Cardinalities
PATIENTHISTORY
PATIENT
PROJECT
EMPLOYEE
PERSON
Mandatorycardinalities
Optional and mandatorycardinalities
Optionalcardinalities
Has
Is assigne
d to
Is married
to
Relationship Cardinality Summary
Mandatory 1 cardinality
Many cardinality (1,2 …m)
Optional (0 or 1) cardinality
Optional (0 or many) cardinality
Unary Relationship
Also called a recursive relationship
PERSON EMPLOYEE
One to manyOne to one
Ismarried
to
Manages
Binary Relationship
A binary relationship is a relationship between instances of two entity types.
PROJECT
EMPLOYEE
SALESORDER
CUSTOMER
ITEM
SUPPLIER
One to one One to many Many to many
Leads PlacesSupplie
s
Ternary Relationship A ternary relationship is a relationship
between instances of three entity types.
PART
suppliesVENDOR CUSTOMER
Attributes
What we want to know about the entity or a relationship
Types:– Derived,– multi-valued, – Composite,– Simple
Example of attributes
EMPLOYEE
Emp-no
Name Address
Skill
Supertypes and subtypes
Benefits:– Presentation– Creativity– Communication– Classifying common patterns– Divide and conquer
Generalisation vs Specialisation
Example of super- and subtypes
EMPLOYEE
Emp-no
Name Address
Skill
HOURLYEMPLOYEE
SALARIEDEMPLOYEE
CONSULTANT
Contract numberStock optionHourly rate
Annual salary
Billing rate
Thinking in Data modelling
Hard Vs Soft ?? Perspective
– Objective vs Subjective– Nature of the organisation
Evaluation of Data modelling
Problem oriented Product oriented
Concep-tual
Structured analysis
Entity relationship modelling
Logical construction of systems
Modern structured analysis
Object oriented analysis
Structured design
Object oriented design
Formal PSL/PSA
JSD
VDM
Levels of abstraction
Stepwise refinement
Proof of correctness
Data abstraction
JSP
Object oriented programming
Advantages of Data modelling
Data model is not computer oriented (agree??) Model understandable by technologist and
users Does not show bias UoD can vary (whole organisation or
department) Readily transformable into other models Different data analysis techniques Data modelling is rule-based
Disadvantages
Does not encourage/support user participation
Your view on the organisation –people or data
The idea that the model is THE model Subjective view One-side ito data Others??
Process modelling view of ISD
Development group
Objectives
Environment
Object system
ObjectsystemChange
process
Hirschheim et al see reading list
Reading for next week
Simsion, G. (1994). Data modeling essentials: Analysis, Design, and Innovation. Van Nostrand Reinhold, USA. Chapter 2, 7 and 10.