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 (Figure 6.2) can be viewed as a data object in the sense that either can be defined in terms of a set of attributes.
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 (3) make reference to another instance in another table.
In the above example, student and college are entities and Stu_ID, Col_ID are attributes.
0 Comments