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The three-schema architecture separates user applications at the topmost level from the physical database at the lowest level.
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Internal Level: Describes the physical storage structure of the database using low-level/physical data models, detailing storage methods and access paths.
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Conceptual Level: Describes the overall structure of the database (entities, data types, relationships, constraints, attributes) while hiding physical storage details.
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External Level: Contains multiple external schemas or views tailored to specific user groups, showing only relevant data and hiding the rest.
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Mapping is the process of transforming requests and results between the three different levels of the architecture.
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Data Independence is the capacity to change the schema at one level without needing to change the schema at the next higher level.
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Logical Data Independence: The ability to modify the conceptual schema (e.g., adding a new attribute) without changing external schemas or application programs.
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Physical Data Independence: The ability to modify the internal schema (e.g., changing file locations or access paths for performance) without changing the conceptual schema.