Download Data Models

IMS1805
Systems Analysis
Topic 3: Doing analysis (cont
from Monday)
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Recap of last lecture
• The importance of understanding the purpose of
analysis
• Some important purposes:
• Organisational;
• Technological;
• Development team
• The purposes behind process models (FDD/DFD)
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2
Agenda
• Aim: To develop further your understanding
of the main purposes for which IS analysis
is done
• To identify the purpose of diagram-based
data analysis techniques in IS
• To work through some examples of the use
of process-oriented and data-oriented
analytical techniques
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2(c) (Continuation of Monday) Why draw data
models?
• Data models enable us to focus on:
• the information which needs to be stored about things
in a system; and
• the information inter-connections which have to
maintained between these things
• Entity-relationship diagrams tell us what things
(entities) we need to store information about,
and what the characteristics are of the
relationships between these things
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4
Data in organisations
• Organisations always have to store data about
things (entities)
• Every entity has many attributes associated with it
• Key data items may be used very widely across an
organisation for a huge range of purposes (often
unrelated) by a wide variety of people
• Inconsistencies in data and data storage between
different parts of an organisation may lead to
confusion and inefficiencies in storages
• Data may be more stable than processes, which
may have to be changed often to meet new needs
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Analysing data in organisations
• We analyse entities:
• To identify all the entities which have to be stored and the
attributes of each one
• To define what linkages (relationships) must be kept
between these entities and what rules apply to these
relationships
• By doing this, we aim to:
• Develop a sound basis for design and construction of
databases to store information consistently and nonredundantly
• Identify areas of shared data/overlapping and conflicting
data needs
• Determine the rules for validating data in databases
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2(c): Data Models: Reminder of E-R basics
(covered in database unit)
• Entities are represented as boxes
• Relationships or connections between entities
(indicating shared data elements) are shown by
lines connecting the entities
• Information about the nature of the relationship
is written next to the connecting line (or in a
diamond-shaped box on the line)
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Why draw an E-R diagram (organisational
purpose)?
• There is always massive overlap and
duplication in data storage needs in different
parts of an organisation
• An organisation-wide E-R Diagram can show
the extent of shared data needs/uses; they
enable development of data structures in which
data can be entered and stored in one place,
but used by many different people to meet
different needs
• The relationships showed in an E-R diagram
allow us to see what connections need to be
made to enable queries between data entities
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Why draw an E-R Diagram (technological
purpose)?
• Relational databases are now the single dominant
form of technology used for storage of data
• Relational databases require the developer to
identify schemas, tables, fields and data validation
rules for entering data into the database
• Database query languages require relationships to
be specified between database tables to permit
queries which connect data across the database
tables
• E-R diagrams provide a framework and ‘rules’ for
database development
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Why draw an E-R diagram (team purpose)?
• Development of a complex system will include
storage of many data items in complex data
structures – multiple databases/database
tables/database fields/etc
• Identifying the entities and their attributes helps
all development team members keep track of
the key data items being recorded by the
organisation
• An E-R diagram provides a central reference
point on system data for all development team
members
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Using Process and Data Models
• See examples discussed in class
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