Database Schema:
The description of a database. Includes descriptions of the database structure and the constraints that should hold on the database.
Schema Diagram:
A diagrammatic display of (some aspects of) a database schema.

Database Instance:
The actual data stored in a database at a particular moment in time. Also called database state (or occurrence).

Logical Data Independence:
The capacity to change the conceptual schema without having to change the external schemas and their application programs.

Physical Data Independence:
The capacity to change the internal schema without having to change the conceptual schema.

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 Data Model: A set of concepts to describe the structure of a database, and certain constraints that the database should obey.

Internal level:
It is the lowest level of data abstraction that deals with the physical representation of the database on the computer and thus, is also known as physical level.
It describes how the data is physically stored and organized on the storage medium. At this level, various aspects are considered to achieve optimal runtime performance and storage space utilization.


Conceptual level:
This level of abstraction deals with the logical structure of the entire database and thus, is also known as logical level.
It describes what data is stored in the database, the relationships among the data and complete view of the user’s requirements without any concern for the physical implementation. That is, it hides the complexity of physical storage structures.
The conceptual view is the overall view of the database and it includes all the information that is going to be represented in the database.

External level:
It is the highest level of abstraction that deals with the user’s view of the database and thus, is also known as view level.

In general, most of the users and application programs do not require the entire data stored in the database. The external level describes a part of the database for a particular group of users.

It permits users to access data in a way that is customized according to their needs, so that the same data can be seen by different users in different ways, at the same time. In this way, it provides a powerful and flexible security mechanism by hiding the parts of the database from certain users, as the user is not aware of existence of any attributes that are missing from the view.

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