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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.

Elements of conceptual design
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)

  • 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.

  • 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.

  • Basic data model operations are used to specify user functional requirements.

  • Actual implementation of database.

  • Physical database design. It includes design of internal storage structures and files.

Overall Database Design Process
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.

A sample network model
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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