In 1975, ANSI-SPARC (American National Standards Institute, Standards Planning And Requirements Committee) defined three-tier for the database systems.
- Internal level
- Conceptual level
- External level
These three levels provide data abstraction means hides the low-level complexities from the end-users.
— This is the lowest level of data abstraction.
— Internal level describes how the data are actually stored on storage devices.
— It is also known as a physical level. It provides an internal view of the physical storage of data.
— This is the next higher level of data abstraction.
— Conceptual level describes what data are stored in the database and what relationships exist among those data.
— It is also known as a logical level.
— This is the highest level of data abstraction.
— External level describes only part of the entire database that a particular end-user concerns.
— It is also known as a view level. It provides end-users with simple interaction with the system.