Introduction to Data Objects Attributes and Relationships
A Data model consists of three different types of interrelated information data objects attributes that describe an object, and differentiate one object from other and the relationships that connect data objects to one another.
Data Objects: By composite information we mean something that has a number of different properties or attributes which can differentiate it from other information. Data objects represent such composite information which must be familiar to or understood by the software. Thus a single value is not sufficient for a valid data object. But a group of such individual values (e.g. colour weight can collectively) form a valid data object.
A valid data object can be of any type a thing an external entity an occurrence a role an organizational unit a place or a structure. These data objects are related to one another. For example teacher and school are towing data objects. The relationship between them can be defined as the teacher teaches in the school.
A data object encapsulates only data. There is no reference to processes or operations that acts on data within a system.
Attributes: An attributes defines the properties of a data object. It also forms the basis on which two data objects can be distinguished from one another. So attributes can be used to:
Name an instance of a data object,
Describe an instance, and
Make reference to another instance of some other data objects (referencing).
One or more attributes can be collectively used as an identifier of a particular entity of a data object. Such collective attributes are known as keys. Referring to a data object student a reasonable identifier must be his school college ID number.
Relationships: Every data object defined in a system are interrelated to one another (may be not to all) in some ways. Relationships are generally bidirectional, which can be either shown as both sided arrows and no arrows on both sides. For example, consider tow data objects medicine and medicine store. Their relationship is shown in fig 4.2.
A set of object relationship pairs can be defined between these two objects:
A medicine store orders medicine.
A medicine store displays medicine.
A medicine store stocks medicine.
A medicine store sells medicine.
A medicine store returns medicine.
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