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Analysis and Design
Data Modeling and Data Requirements
Last chapter discusses about one part of the conceptual design process, the functional
model. The other is the data model, which discusses the data related design issues
of the system. See fig 7.1. The data model focuses on what data should be stored
in the database while the function model deals with how the data is processed.
In this chapter, we'll look into details of data modeling.
Fig 7.1 - Elements of conceptual design
We have already discussed the Data Flow Diagrams, which make the foundation
of the system under development. While the system is being studied, the physical
DFDs are prepared whereas at the design phase, the basic layout of the proposed
system is depicted in the form of a logical DFD. Taking this DFD as the basis
the system is further developed. Even at the Data Modeling phase, the DFD can
provide the basis in the form of the data flows and the Data Stores depicted in
the DFD of the proposed system. The Data Stores from the DFD are picked up and
based on the data being stored by them the Data Model of the system is prepared.
Prior to data modeling, we'll talk of basics of database design process. The
database design process can be described as a set of following steps. (Also see
figure below 7.2 Overall Database Design Process)
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Requirement collection: Here the database designer interviews database users.
By this process they are able to understand their data requirements. Results of
this process are clearly documented. In addition to this, functional requirements
are also specified. Functional requirements are user defined operations or transaction
like retrievals, updates, etc, that are applied on the database.
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Conceptual schema: Conceptual schema is created. It is the description of data
requirements of the users. It includes description of data types, relationships
and constraints.
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Basic data model operations are used to specify user functional requirements.
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Actual implementation of database.
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Physical database design. It includes design of internal storage structures
and files.
Fig 7.2 - Overall Database Design Process
In this chapter, our main concern is data model. There are various data models
available. They fall in three different groups.
Object-Based
Logical Models
Object-based logical models are used in describing data at the logical and
view levels. The main characteristic of these models is that they provide flexible
structuring capabilities and allows data constraints to be specified explicitly.
Many different models fall into this group. They are following.
- Entity-relationship model
- Object-oriented model
In this chapter, we’ll discuss Entity-Relationship model in detail. The
object-oriented model is covered in the next chapter.
Record-Based
Logical Models
Records-based logical models are used in describing data at the logical and
view levels. They are used to specify the overall logical structure of the database
and to provide a higher-level description of the implementation.
In record-based models, the database is structured in fixed-format records
of several types. Each record type defines a fixed number of fields, or attributes,
and each field is usually of a fixed length. The use of fixed-length records simplifies
the physical-level implementation of the database.
The following models fall in this group.
Relational Model
This model uses a collection of tables to represent both data and relationship
among those data. Each table has multiple columns, and each column has a unique
name. Figure shows a simple relational database.
Fig 7.3 - A sample relational model
Network Model
In network database, data is represented by collection of records, and relationships
among data are represented by links. The records are organized as a collection
of arbitrary graphs. Figure 7.4 represent a simple network database.
Fig 7.4 - A sample network model
Hierarchical Model
The hierarchical model is similar to the network model. Like network model,
records and links represent data and relationships among data respectively. It
differs from the network model in that the records are organized as collections
of trees rather than arbitrary graphs. Fig 7.5 represents a simple database.
Physical Data
Models
Physical data models are used to describe data at the lowest level. A few physical
data models are in use. Two such models are:
- Unifying model
- Frame-memory model
Physical data models capture aspects of database-system implementation.
See Also
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