Transcript Modelling the `Real` World

44220: Database Design & Implementation
Modelling the ‘Real’ World
Ian Perry
Room: C49
Ext.: 7287
E-mail: [email protected]
http://itsy.co.uk/ac/0506/sem2/44220_DDI/
Last Lecture ended with this …
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This Lecture:
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Explores the complex idea of ‘Data Modelling’.
44220: Database Design & Implementation: Modelling the ‘Real’ World
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What is this?
Ian Perry
44220: Database Design & Implementation: Modelling the ‘Real’ World
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And this?
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Any of the family of aquatic birds,
especially those having short legs,
webbed feet, and a broad blunt bill.
Collins Concise English Dictionary
Ian Perry
44220: Database Design & Implementation: Modelling the ‘Real’ World
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And this?
dTw
 k 1 TE  TW    k 2V  k 3F  TA  TW 
dt
Ian Perry
44220: Database Design & Implementation: Modelling the ‘Real’ World
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And this?
Duck
• What do they have in common?
• They are ALL, depending upon your
information requirements, perfectly
adequate models of a Duck.
Ian Perry
44220: Database Design & Implementation: Modelling the ‘Real’ World
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So, what is a Model?
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Always remember that;
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They are:
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Models ARE NOT the Real thing.
the appearance of reality.
an analogue of the real world.
a simplified representation of reality.
the abstraction of meaning (the semantics).
We build models for a purpose, so;
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being clear as to the purpose of a model is
the key to success.
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Every-day Models
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Model Duck:
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Model Aeroplane:
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Purpose; to show shape, colour, size, etc.
Purpose; to show general structure,
identification of parts, flight
characteristics, etc.
Data Model:
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Purpose; the representation of objects of
interest to an enterprise, allowing data to
be structured (i.e. given meaning) and
manipulated (for specific purposes).
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The Data Abstraction Process
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Requires us to focus on the critical
aspects of the real world’s richness.
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All models require decisions about:
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no model is complete!
what to include & what to exclude?
These decisions are represent someone’s
view of a particular reality, i.e.;
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what is important, and MUST be included.
what is not important, and can be left out.
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Data Models in the Plural
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The complexity of the database design
process means that we must use a
hierarchy of data models:
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Conceptual Data Model
Logical Data Model
Physical Data Model
Successful database design requires;
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an orderly, and rigorous, progression
through this ‘Data Modelling Stack’.
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The ‘Data Modelling Stack’
Conceptual Overview of things
Logical
Physical
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that are perceived to
be of ‘interest’ in the
‘real’ world.
Data elements & the
relationships between
those elements in a
tabular form.
Actual data held in a
database & the
means to manipulate
that data.
Model of the
Business System.
(ER Model)
Model of Data
Storage Theory
(Db Schema)
Physical
Implementation
(RDBMS)
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Conceptual Data Model
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Initial ‘view’ of the Objects of interest;
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An integrated ‘view’ of the whole;
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which is compromise free AND Software &
Hardware independent.
Conceptual Data Modelling, is all about;
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their properties, relationships, semantics.
‘capturing’ WHAT real-world Objects
MUST be included, i.e. to suit a particular
purpose.
We will build Conceptual Models using:
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Entity Relationship (ER) Diagrams
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Logical Data Model
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Logical Data Modelling, is all about;
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We will build these using:
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‘deciding’ HOW to best represent the
properties of the Objects of interest, and
the relationships between them.
Database Schema
A Database Schema:
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defines the single, integrated, data
collection that is the whole database.
within the constraints of a chosen/imposed
theoretical framework.
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Theories of Logical Modelling
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Several to choose from, e.g.:
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Each Logical Modelling Theory comes with its
own:
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Hierarchical, Relational, Object-based, etc.
Data Definition Language (DDL)
Data Manipulation Language (DDM)
Data Query Language (DQL)
Software availability tends to influence the
Logical Modelling Theory chosen here:
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As we will be using Microsoft Access (a Relational
DataBase Management System), our logical choice
must be Relational.
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Physical Data Model
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Physical Data Modelling, is all about;
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This process may be influenced by;
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‘mapping’ a Logical Data Model onto a specific
Physical Storage System.
both Software specific & Hardware specific
considerations.
i.e. one may have to compromise the Logical
Data Model;
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in order to make it ‘fit’ the Software (in our
case Microsoft Access) that will be used build,
maintain & manipulate the Physical Data Model.
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Database Design & Development
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Analyse
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Develop
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a logical data model (constrained by a particular
database theory); from the conceptual data model.
Implement
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a conceptual data model; from the above analysis.
Develop
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a business situation; in order to ‘discover’ the purpose.
a physical data model (constrained by software
availability); based on the logical data model.
Manipulate
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i.e. test the physical data model; to prove it ‘suits’ the
purpose discovered by the original analysis.
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DDI Assessment Method
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Assignment 1 (50%) [01 Mar, 2006]
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Develop; an ‘appropriate’ Conceptual Data
Model for a Case Study organisation.
Develop; a ‘robust’ Logical Data Model
based on the Conceptual Data Model.
Assignment 2 (50%) [28 Apr, 2006]
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Implement; a ‘accurate’ Physical Data
Model based on the Logical Data Model.
Manipulate; the Physical Data Model to
‘prove it can’ Answer Specific Questions
from the Case Study.
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This Week’s Workshop
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In this Workshop we will begin to explore some of the
decisions we must make as we attempt to construct a
model of ‘real’ world ‘things’.
Working alone, or in a small team, consider the
following half dozen ‘things’ that exist in the ‘real’
world.
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For each of the above;
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list as many as you can imagine (try to be as creative &
inventive as you can) of the ways that it might be modelled.
For each way of modelling the above ‘things’;
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University, Computer, Bank, Shoe Shop, Doctor’s Surgery,
Sports Activity.
indicate who might regard this form of model as a meaningful
and useful abstraction that captures its essential aspects.
DO NOT arrive at the workshop un-prepared!
Ian Perry
44220: Database Design & Implementation: Modelling the ‘Real’ World
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