In 1971, DBTG(DataBase Task Group) realized the requirement for a two-level approach having views and schema and afterward, in 1975, ANSI-SPARC realized the need for a three-level approach with the three levels of abstraction comprises of an external, a conceptual, and an internal level.
The three-level architecture aims to separate each user’s view of the database from the way the database is physically represented.
- External level:
It is the view how the user views the database. The data of the database that is relevant to that user is described at this level. The external level consists of several different external views of the database. In the external view only that entities, attributes, and relationships are included that the user wants. The different views may have different ways of representing the same data. For example, one user may view name in the form (firstname, lastname), while another may view as (lastname, firstname).
- Conceptual level:
It is the community view of the database and describes what data is stored in the database and represents the entities, their attributes, and their relationships. It represents the semantic, security, and integrity information about the data. The middle-level or the second-level in the three-level architecture is the conceptual level. This level contains the logical structure of the entire database, it represents the complete view of the database that the organization demands independent of any storage consideration.
- Internal level:
At the internal level, the database is represented physically on the computer. It emphasizes the physical implementation of the database to do storage space utilization and to achieve the optimal runtime performance, and data encryption techniques. It interfaces with the operating system to place the data on storage files and build the storage space, retrieve the data, etc.
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