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 programmer's (pilot's) view. Many subschemas can be derived from one schema, just as different pilots visualize different views of a landing approach, although all (it is hoped) arrive at the sheduled 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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