Introduction to Data Objects, Attributes and Relationships
When data modelling is used as a requirement analysis technique the analysis begins by creating models for objects. A data objects can be defined as:
It is a representation of information that must be understand by the software.
It is defined almost in the same manner as the objects of the OOA. Data objects can be external entities things occurrences or events roles organizational units, places or structures. It can be better understood with the help of an example:
A person busies a scoter.
Here person and scooter are the data objects. So the attributes of each one and a relationships between both are defined. In this way they are linked with each other. This example is shown in fig 7.30
As we can see in fig 7.25, that each objects has some attributes and they are connected with each other via a link. These relationships are always defined by the context of the problem that is beings analyzed.
A data objects encapsulate the data so there is so reference within objects to operations that act on the data. That why are objects can be represented as a table 7.3.
In table 7.3, various attributes of data objects scooter are defined.
In the table the attributes of data scooter are specifically named and their respective entries are also made. Id number is an identifier model and id number represent the naming attributes. Body type and color are descriptive attributes and the own of the scooter is the referential attributes ties one data objects to another.
So, this attributes makes the link between scooter and the person who buss it. The body of the table 7.3 represents specific instances of the data objects. For example model LA400, id number B23 of body type sports with color blue and which is bought by B, represents an instance of the data object scooter.
There are some major characteristics to define objects attributes. They are described below:
Additionally one or more of the attributes must be defined as an identifier where the identifier attributes becomes a key attributes when we want to find an instance of the data objects. The values of the identifier must be unique. Attributes for data objects are determined inexactly the same way as suggested for attributes defined as part of objects oriented analysis.
To reduce the redundancy and to provide consistency in table these data objects tables are formalized by applying a set of normalization rules that will result in a relational model for the data. These normalization rules are given below:
1. A given instance of a data objects has one and only one value for each objects.
2. Attributes represent elementary data items they should contain no internal structure.
3. When more than one attributes is used to identify a data objects be sure that descriptive and referential attributes represent a characteristics of the entire objects and not a characteristic of some things that would be identified by only part of the identifier.
All non identifier attributes must represent some characteristic of the instance named by the identifier of the data objects and describe some other attributes that is not an identifier.
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