Database Analysis and Design Methods Analysis and Design Aims

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Database Analysis and Design Methods

    Analysis and Design Aims

    Questions to be addressed                         Analysis and Design Aims
                                                      To produce a robust, exible database that
    Major Steps in Analysis and Design                will provide reliable support for the di erent
                                                      and evolving applications provided by an
    Database Analysis and Design                      information system.
    Techniques

    Database Analysis and Design Methods


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Questions to be addressed                             Major Steps in Analysis and Design
    What will be stored in the database?               1. Data Analysis
    How will the data evolve in time?                  2. Conceptual Design
    How will the data be represented                   3. Process Design
        at the conceptual level, and
        in terms of a particular database              4. Physical Design
        management system?                                a Structural Design

    What processing will be required?                     b Implementation Design


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                                                 Database Analysis and Design Methods
Database Analysis and Design
Techniques                                           Organised application of techniques

    Data ow modelling                                Traceable, reproducible, manageable
                                                     processes
    Entity-Relationship modelling
                                                     Examples:
    Relational data modelling                            Structured Systems Analysis De
                                                         Marco
    Combined techniques                                  Information Engineering
                                                         MERISE
    Entity-Event modelling
                                                         SSADM
    Database Tuning                                      OMT Rumbaugh
                                                         :::

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Techniques used in SSADM and OMT
SSADM and OMT combine di erent                   Data ow Modelling
techniques to produce an overall system
design.
                                                     Current Physical Data ow Model
An important feature of SSADM and OMT
is that they both make use of three di erent         Current Logical Data ow Model
models to describe di erent aspects of the
data in an information system.
                                                     Required Logical Data ow Model
    Data ow Model
                                                 The main aim of data ow modelling is to
    Entity-Relationship Model                    identify events and functions of the required
                                                 system.
    Entity-Event Model

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Current Physical Data ow Model

    Describes the movement of data-bearing
    entities, including documents, labels on
    boxes, name badges etc.                       Hoarders and Collectors Ltd.
                                                  Current Physical Context Diagram
    Describes the movement of physical
    entities                                                         Purchase Orders
                                                                         goods
                                                                                           0
                                                                                                                     Sales Orders
                                                                                               Hoarders and             goods
                                                      Suppliers                                                                            Customers
                                                                                               Collectors Ltd.
                                                                     Delivery Notes                                      Delivery Notes

    Describes how these real entities are
    currently processed

    and how they are stored


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Current Logical Data ow Model

    Describes how data moves through the          Hoarders and Collectors Ltd.
    current system                                Current Logical Context Diagram

    as well as the data transforming
                                                                                       0
                                                                  Purchase Orders                                Sales Orders

    processes that are currently enacted           Suppliers

                                                                  Delivery Notes
                                                                                           Hoarders and
                                                                                           Collectors Ltd.       Delivery Notes
                                                                                                                                          Customers




    and how data items are currently stored




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                                                                                    Hoarders and Collectors Ltd.
                                                                                    Current Logical Level 2 Diagram
                                                                                    Expansion of Accept Delivery"
Hoarders and Collectors Ltd.
Current Logical Level 1 Diagram                                                                           Purchase
                                                                                                          Order


                                                                                                                         Recd.
                    1                            3
                                                                  S.O                                    2.1
             P.O
 Suppliers                                       Accept                 Customers   Delivery Note
                    Order                                                                                 Check                     Checked
                                                 Order                              (from Suppliers)                                Delivery Note
                    Goods             Qty.   Qty.                                                         Delivery
                                       in     in
                        Purchase     Stock   Stock        Sales                                                   Filled Purchase
                                                          Order                                                   Order
                        Order         D1 Stock File                                                                                 2.2
                                      Qty.    Qty.                                                     D2 Filled P.O.s              Update          Goods received
                                     Recd.    Sold
                                                                                                                                    Stock File      (to stock file)
                    2                             4
             D.N.                                                 D.N
                        Accept                    Fill                  Customers
 Suppliers
                        Deliveries                Order



                                                                                           Who looks at the lled orders stored in
                                                                                           D2?

                                                                                           Where are un lled orders stored?
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                                                                                    Creating the Current Physical Data ow
                                                                                    Model
Required Logical Data ow Model
                                                                                           The model consists of
      Describes the required or proposed
      data movement                                                                              Data ow Diagrams
                                                                                                 Supporting Documentation
      as well as the data transforming
      processes that are to be enacted
                                                                                           The diagrams are constructed using
                                                                                           basic symbols
      and how data items will be stored
                                                                                           and are organised as a levelled hierarchy.



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Levelled Data ow Diagrams
                                                                  Data ow Diagrams Basic Symbols
         A
                             0

                                                    B



                                                                                    Process


                                                    2
     A
                         1


                                     D1                                             Datastore

                             3



                                                B                                   External System



                                                                                    Dataflow
                                          3.2


                   3.1

                                                          B                         Physical flow




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Supporting Documentation                                          Process Descriptions
As well as the data ow diagrams, a data ow                            Narrative Prose
model includes                                                        should not be too long; about half an
                                                                      A4 page per lowest level process.
    descriptions of the lowest level processes
                                                                      Decision Tables
    descriptions of the structure and content                         useful when alternative actions are taken
    of the datastores                                                 depending on clearly de ned
                                                                      circumstances.
    descriptions of the data ows
                                                                      Structured English
                                                                      makes use of a limited vocabulary and
    descriptions of the external entities                             syntax.


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                                                                                Data ow and Datastore Descriptions
                                                                                These are usually represented as data dictionary
                                                                                entries.
Structured English example                                                      Example data dictionary entry for Delivery
                                                                                Note
    ORDER GOODS                                                                     Name: Delivery Note
                                                                                    Source: Fill Order Process 4
                                                                                    Destination: Customer External Entity
                                                                                    Contents: Customer id.
    1. for each kind of stock
                                                                                              Delivery address
       1.1 check quantity in store                                                            Goods' description
                                                                                                    code          
       1.2 if quantity in stock is less than                                                        description    repeats
              reorder level                                                                         quantity      
                                                                                              Delivery Note Number
           then issue purchase order                                                          Date



                                                                                Data Dictionary entries may also be created for
                                                                                processes and external entities.


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Case Study a Video Library                                                      Transforming Current Physical Data ow
See Downs, Clare and Coe, SSADM: Application and                                Diagrams to obtain Current Logical
Context, 2nd Edition, Prentice-Hall 1992, section 3.2                           Data ow Diagrams
Context Data ow Diagram                                                         A current physical data ow model shows
                                                                                how data is processed in an existing
                      Library
                                                                                information system.
                     Committee

             Video details   Video       Enquiry                                The current physical model is further
    Please return                        Membership card                        analysed and transformed to produce a
                                                                                current logical data ow model.
                    0 Context
            Video                        Video             Borrower
 Borrower           GVLC                available
                                        available
                Change of
                address
                               New
                               member
                                         Reservation
                                                                                The current logical data ow model
                    Membership                                                  describes what data processing is carried out
                    Secretary
                                                                                in the current system, regardless of how
see Downs, Clare and Coe, gures 3.8 and 3.10                                    that system is implemented.

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Transforming the Current Physical
Data ow Model to obtain the Current
Logical Data ow Model                                   Determining the Logical Datastores
                                                                 Remove duplicate datastores
Using the current physical data ow                                 logically, each item of data is stored only
diagrams,                                                          once
                                                                   in real life and curent physical DFDs it is
                                                                   often duplicated
 1. specify logical datastores,                                    duplication is discovered by crossreferencing
                                                                   entities with datastores
                                                                   aim for a situation were
 2. write logical descriptions of lowest level                       each entity corresponds to exactly one
    processes and data ows,                                           datastore
                                                                     each datastore corresponds to one or more
                                                                      entities
 3. group lowest leve processes to form                          Eliminate transient datastores
    higher level processes until level 1                            datastores that exist in order to implement
    processes have been obtained, and                               the current system; e.g. a le where video
                                                                    cards are stored before being moved to the
                                                                    loans le.
 4. draw a logical context diagram.
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                                                        Current Logical DFD for GVLC
                                                        reservations
Making the lowest level processes and data ows
logical                                                     Borrowers                                        Borrowers


    Ensure that each process transforms some data                   Reservatoin                                         Please Return

       if data passes through a process without
       being transformed, replace the process by a               3.4                                      3.3
       data ow, or let the data bypass the process               Record
                                                                 Reservation
                                                                                                          Request

       remove minor retrievals
                                                                                                          Return


       abolish processes that were required to                   Reservation details          Reservation details

       maintain consistency between duplicate
       datastores
                                                                               D4      Reservations
                                                                                       Reservation
                                                                                        details

    combine processes that are always performed                                     3.2
    together                                                  video
                                                             available
                                                                                    Notify
                                                                                                      Reservation details
                                                                                    Availability

    remove duplicate processes
                                                                                     video

    remove physical ows, references to physical
                                                                                    available


    locations and references to forms currently used
    rather let these refer to the data items           2                                                       1

    embodied in the forms
                                                                                    Borrowers
                                                                                                                Issue
                                                        Return



                                                        compare with current physical DFDs, Downs, Clare
                                                        and Coe
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Grouping processes to form higher level
processes     :::
                                                  Summary
all the way back to level 1
                                                      transform the current physical DFDs to produce
                                                      current logical DFDs
    a bottom up restructuring of the                  use a technique called logicalization", which
    hierarchy of levelled DFDs                        results in logical
                                                          datastores,
    aim to minimize data ow between                       bottom level processes, and
    processes at the top level by grouping                levelling hierarchy.
    processes that use the same data




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Developing the Required Logical                   Developing the Required Logical
Data ow Model                                     Data ow Model

The current logical data ow modelling              1. Identify the domain of change
describes the data processing carried out by
the existing information system.                   2. Create data ow models for a selection
The required logical data ow model                    of proposed new systems
describes the processing that will be carried
out by the new system that will replace the        3. Assist the client in choosing from
existing information system.                          amongst the best of these proposalss




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Identify the Domain of Change
                                                      Domain of Change shaded
    include any bottom level process that is
    likely to be changed in the new system
    for example, by being partly or fully
    automated, eliminated, or done in a
    di erent way

    if a process will not change, it lies
    outside the domain of change

    there may be several disconnected areas
    from the current logical DFDs that lie
    within the domain of change

    but the domain of change is a single
    item
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The context diagram for the required                  Level 1 proposals for the required
system                                                system
    replace the entire domain of change by a              several level 1 diagrams are invented
    single process; this is the context
    diagram for the required system
                                                          fresh creative views are essential
    any data ow that crossed the boundary
    of the domain of change appears in the                problems and requirements are taken
    new context diagram                                   into account




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Guidelines for new level 1 DFDs
                                                         Business System Options
    partition to minimise interfaces between
    processes                                                options for the new system based on the best
                                                             level proposals
    name processes for the data transformations
    they achieve                                             usually between two and six options are prepared
    and name data ows for the data they carry                each includes
    don't include too many processes in a DFD                   a level 1 DFD
                                                                an entity relationship model or logical data
    include every data ow                                       model
    make sure every process transforms some data                narrative outlining main features
                                                                costs, bene ts and e ects
    use a data dictionary
                                                             existing systems should always be considered
    be prepared to scrap diagrams




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Business System Options for the GVLC
1. A PC based package built using a                      Business System Options for the GVLC
standard dbms like Access
                                                         2. A specially tailored system running on
                                                         a client server network
    low cost
                                                             expensive
    speedy implementation
                                                             slower time to installation
    comprehensive, exible and clear user
    enquiry facility
                                                             immense capacity for growth and
                                                             extension
    some limitations on the volume of data
    that can be held


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Business System Options for the GVLC              Resource Flow Diagrams
3. A web site
                                                        model physical movement of goods
    immediate distributed access
                                                        used as a preliminary step
    capacity to link to other systems
                                                                             1 Post Room
                                                                    Tapes                        Tapes
                                                      Supplier                                             New Tapes
                                                                             Receive goods

    some security hazards                                                                           etc.
                                                                                                                 Tapes




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                                                  Document Flow Diagram
Document Flow Diagrams                                               Expenditure Report
                                                                                                 Library
                                                                                                 Committee

    model external entities and data ows              Accounts
                                                                                           Suggestions list
                                                             Authorised invoice                               New videos report


    and movement of documents                                                     Acquisitions
                                                                                  Officer
                                                                                                           Video
                                                                                                           details
                                                                                                                     Issue discharge
                                                                                                                      system

    help in identifying the system boundary
                                                                   Order
                                                                            Invoice


                                                        Supplier




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Description: Database Analysis and Design Methods Analysis and Design Aims