The architecture is divided into three levels, known as the internal, conceptual, and external level , respectively:
1. The internal level has an internal schema is the one closest to physical storage. i.e., it is the one concerned with the way the data is physically stored and describes the physical storage structure of the database. This is known as the physical level.
2. The external level includes a number of external schema or use views is the one closest to the users - that describe the view of different user groups i.e., it is the one concerned with the way the data is seen by individual users. This is known as the user logical level.
3. The conceptual level has a conceptual schema is a high-level description of the whole database. The conceptual schema hides the details of physical storage structures and concentrates on describing entities, data types, relationships, user operations and constraints. This is known as community logical level.
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Three-schema architecture supports the following DBMS characteristics:
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- Program-data independence
- Support for multiple views of the data
- Defines DBMS schemas at three levels:
(i) Internal schema at the internal level to describe data storage structures and access paths that typically uses a physical data model.
(ii) Conceptual schema at the conceptual level to describe the structure and constraints for the whole database that uses a conceptual or an implementation data model.
(iii) External schema at the external level to describe the various user views. Usually uses the same data model as the conceptual level.
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One important characteristic of the database approach is the insulation of programs and data. Users can define two types of data independence:
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1. Logical data independence: It is the capacity to change the conceptual schema without having to change external schemas or application programs.
2. Physical data independence: It is the capacity to change the internal schema without having to change the conceptual (or external) schemas. Changes to the internal schema may be needed because some physical files had to be reorganised.
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