Software Engineering-Data Modelling

Data modeling answers a set of specific questions that are relevant to any data processing application. What are the primary data objects to be processed by the system? What is the composition of each data object and what attributes describe the object? Where do the the objects currently reside? What are the relationships between each object and other objects? What are the relationships between the objects and the processes that transform them?

To answer these questions, data modeling methods make use of the entity relationship diagram. The ERD,enables a software ngineer to identify data objects and their relationships using a graphical notation. In the context of structured analysis, the ERD defines all data that are entered, stored, transformed, and produced within an application.

The entity relationship diagram focuses solely on data (and therefore satisfies the first operational analysis principles), representing a "data network" that exists for a given system. The ERD is especially useful for applications in which data and the relationships that govern data are complex. Unlike the data flow diagram, data modeling considers data independent of the processing that transforms the data.

Data Objects, Attributes, and Relationships

The data model consists of three interrelated pieces of information: the data object, the attributes that describe the data object, and the relationships that connect data objects to one another.
Data objects. A data object is a representation of almost any composite information that must be understood by software. By composite information, we mean something that has a number of different properties or attributes. Therefore, width (a single value) would not be a valid data object, but dimensions (incorporating height, width, and depth) could be defined as an object.

A data object can be an external entity (e.g., anything that produces or consumes information), a thing (e.g., a report or a display), an occurrence (e.g., a telephone call) or event (e.g., an alarm), a role (e.g., salesperson), an organizational unit (e.g., accounting department), a place (e.g., a warehouse), or a structure (e.g., a file). For example, a person or a car can be viewed as a data object in the sense that either can be defined in terms of a set of attributes. The data object description incorporates the data object and all of its attributes.

Data objects (represented in bold) are related to one another. For example, person can own car, where the relationship own connotes a specific "connection” between person and car. The relationships are always defined by the context of the problem that is being analyzed.

A data object encapsulates data only—there is no reference within a data object to operations that act on the data.
Attributes. Attributes define the properties of a data object and take on one of three different characteristics. They can be used to (1) name an instance of the data object, (2) describe the instance, or (3) make reference to another instance in another table. In addition, one or more of the attributes must be defined as an identifier—that is, the identifier attribute becomes a "key" when we want to find an instance of the data object. In some cases, values for the identifier(s) are unique, although this is not a requirement. Referring to the data object car, a reasonable identifier might be the ID number.

The set of attributes that is appropriate for a given data object is determined through an understanding of the problem context. The attributes for car might serve well for an application that would be used by a Department of Motor Vehicles, but these attributes would be useless for an automobile company that needs manufacturing control software. In the latter case, the attributes for car might also include ID number, body type and color, but many additional attributes (e.g., interior code, drive train type, trim package designator, transmission type) would have to be added to make car a meaningful object in the manufacturing control context.

Relationships. Data objects are connected to one another in different ways. Consider two data objects, book and bookstore. . A connection is established between book and bookstore because the two objects are related. But what are the relationships? To determine the answer, we must understand the role of books and bookstores within the context of the software to be built. We can define a set of object/relationship pairs that define the relevant relationships. For example,

A bookstore orders books.
A bookstore displays books.
A bookstore stocks books.
A bookstore sells books.
A bookstore returns books.

The relationships orders, displays, stocks, sells, and returns define the relevant connections
between book and bookstore.
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