Three Schema Architecture | Three Level of Abstraction | Database Management System

What is Three Schema Architecture?

Schema means **structure.

Structure means the way we organize the data we are storing (e.g., Student, Railway).

Example:

Raw Data → 1, A, 20

Same data in a table:

Roll	Name	Age
1	A	20

Here, Roll number is an integer (int) value.

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There are three levels of data abstraction:

1. Conceptual Schema (Designer - Defines How Data is Stored - ER Model)

• Defines the structure of the data.
• Example: What tables and attributes will exist in the student database?

2. Physical Schema (Administrator - Manages Database Storage)

• Defines how and where the data will be stored.
• Example: Database administrators decide how student records are stored in MySQL, Oracle, or another database system.

3. External Schema (User - View Level - Limited Access)

• Users can see and access only limited data but cannot modify or see the full database.
• Example: A student can view their own exam results on the university website but cannot see other students' data.

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Types of Database Files:

1. Data Files – Store actual data (e.g., student records).
2. Log Files – Keep track of changes made to the database.
3. Index Files – Speed up searching and retrieving data.