3.1 The Three-Schema Architecture

Also known as the ANSI/SPARC Architecture, its primary goal is to separate the user applications from the physical database. This provides a high degree of data independence.

The Three Levels:

1. Internal Level (Physical Level):

• The lowest level of abstraction.
• Describes how the data is actually stored on the disk (blocks, indexes, hashing).
• Uses a physical data model.

1. Conceptual Level (Logical Level):

• The middle level of abstraction.
• Describes what data is stored in the database and what relationships exist among those data.
• It describes the entire database structure for the whole community of users.
• Focuses on entities, data types, relationships, and constraints.

1. External Level (View Level):

• The highest level of abstraction.
• Describes only part of the entire database for a specific group of users.
• Each external schema describes the database "view" for one or more user groups.

3.2 Mapping Between Levels

The DBMS is responsible for "mapping" between these levels.

• External/Conceptual Mapping: Defines the correspondence between a particular external view and the conceptual schema.
• Conceptual/Internal Mapping: Defines how the conceptual records and fields are represented at the internal level.

3.3 Client/Server Architecture

Modern DBMS systems are typically organized in a client/server manner to distribute the workload.

Evolution of Architectures:

• Client Tasks: User interface, data entry, report formatting.
• Server Tasks: Query processing, optimization, transaction management, storage.