The three-level schema architecture is a conceptual model used in database management systems (DBMS) to describe the overall design of a database system. This model consists of three levels or layers of abstraction, each representing a different perspective of the database system:
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External or User Level: The external or user level is the level closest to the end-users, and it represents the way users perceive the data. This level defines the external schema, which describes how the data appears to the individual users or applications. In other words, it defines the portion of the database that is relevant to a particular user or application. Each user or application can have their own external schema, which is tailored to their specific needs and requirements. The external schema is a view of the global schema, which is defined at the conceptual level.
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Conceptual or Logical Level: The conceptual or logical level is the level of abstraction that describes the entire database in terms of a set of integrated entities and relationships. It defines the logical schema, which describes the structure of the entire database as a whole. The logical schema is independent of any physical implementation details, such as storage devices or file systems, and is designed to be understandable by both users and application developers. The logical schema is a view of the internal schema, which is defined at the physical level.
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Internal or Physical Level: The internal or physical level is the lowest level of abstraction, and it describes the way data is stored on the physical storage devices, such as hard disks or solid-state drives. It defines the physical schema, which describes how the data is physically arranged and stored on the storage devices. The physical schema is designed to optimize the performance and efficiency of the database system, and it is often hidden from both users and application developers. The physical schema is a view of the actual data stored in the database.
The three-level schema architecture provides a clear separation of concerns between the different layers, which allows for easier maintenance, scalability, and security of the database system. It also enables the database system to be easily modified or extended without affecting the external schema or the end-users.
