data abstraction levels in data Base

Data Abstraction

The major purpose of a database system is to provide users with an abstract view of the system.
The system hides certain details of how data is stored and created and maintained
Complexity should be hidden from database users.
There are several levels of abstraction:
1. Physical Level:
  - How the data are stored.
  - E.g. index, B-tree, hashing.
  - Lowest level of abstraction.
  - Complex low-level structures described in detail.
_ Logical level /Conceptual Level:
- Next highest level of abstraction.
- Describes what data are stored.
- Describes the relationships among data.
- Database administrator level.

View Level:

Highest level.
Describes part of the database for a particular group of users.
Can be many different views of a database.
E.g. tellers in a bank get a view of customer accounts, but not of payroll data.

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Fig. [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] (figure 1.1 in the text) illustrates the three levels. </LI>

Figure 1.1: The three levels of data abstraction