SAD-Logical And Physical View Of Data

Logical and physical views of data:
 
In data base design, several views of data must be considered along with the persons who use them. In addition to data structuring, where relationships are reflected between and within entities, we need to identify the application program’s logical views of data within an overall logical data structure. The logical view is what the data look like regardless of how they are stored. The physical view is the way data exists in physical storage. It deals with how data are stored, accessed or related to other data in storage.

Schemas and subschemas:

The schema is the view that helps the DBMS decide what data in storage it should act upon as requested by the application program. An example of a schema is the arrival and departure display at an airport. Scheduled flights and flight numbers  (schema) remain the same, but the actual departure and arrival times may vary. The user’s view might be a particular flight arriving or departing at a scheduled time. How the flight actually takes off or lands is of little concern to the user. The latter view is of  subschema. It is a programmers view. Many subschemas can be derived from one schema, just as different pilots visualize different views of a landing approach, although all arrive at the scheduled time indicated on the CRT screen display (Schema).

Different application programmers visualize different subschemas. The relationships  among  the schema, subschema and physical structure are provided by the software.
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