The three levels of abstraction differ mainly in what they describe, who uses them, and how much detail they expose. The comparison is given below:
| Basis | Physical Level | Conceptual (Logical) Level | View Level |
|---|---|---|---|
| Also called | Internal Level | Logical Level | External Level |
| Describes | Describes how data is physically stored on storage devices, such as files, blocks, indexes, and storage locations. | Describes what data is stored in the database and the relationships among different data items or tables. | Describes only the part of the database that is relevant to a particular user or user group. |
| Level of detail | Lowest level – It provides the most detailed information about physical storage, such as files, indexes, and blocks. | Middle level – It describes the overall structure of the database, including tables, relationships, and schema. | Highest level – It provides a simplified and user-specific view of the database without showing unnecessary details. |
| Used by | Mainly used by DBAs and system designers who manage the physical storage and performance of the database. | Mainly used by DBAs and some application programmers who work with the overall database structure. | Mainly used by end users and application programmers who need to access only specific data. |
| Number of instances | There is one physical schema that describes how the entire database is physically stored. | There is one conceptual schema that describes the complete logical structure of the database. | There can be many external views, with one or more views created for different users or user groups. |
| Purpose | Its purpose is to ensure efficient storage and fast retrieval of data by deciding how data is physically organized and accessed. | Its purpose is to represent the overall logical structure of the database, including data, relationships, and constraints, without dealing with physical storage details. | Its purpose is to simplify interaction with the database by showing users only the required information. It also improves security and privacy by hiding unnecessary or sensitive data. |