Week 2

2.2 The Three-Schema (ANSI/SPARC) Architecture

The ANSI/SPARC three-schema architecture provides a framework that separates the user's view of data from the physical storage of data. This separation is called data independence.

The three schemas (levels) are:

Schema Level	Also Called	Description
External Schema	View Level / User View	What each group of users sees. Different users may see different views of the same data. E.g., a bank teller sees customer balances; the auditor sees transaction logs.
Conceptual Schema	Logical Level	The complete logical structure of the database all tables, relationships, and constraints. Independent of physical storage. E.g., the complete bank database structure with all tables defined.
Internal Schema	Physical Level / Storage Level	How data is physically stored on disk, file organisations, indexes, storage structures. E.g., data stored in B-tree indexed files on SSDs in the data centre.

Three-Schema Analogy: Think of a Nigerian government office building. The INTERNAL level is the building's structure, beams, pipes, electrical wiring (users never see this). The CONCEPTUAL level is the official floor plan where each department is located. The EXTERNAL level is what each visitor sees the reception desk, their specific meeting room, each visitor has their own 'view' of the building.