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We know that three view-levels are described by means of three schemas. These schemas are stored in the data dictionary. In DBMS, each user
refers only to its own external schema. Hence, the DBMS
must transform a request on. a specified external schema into a request against conceptual schema, and then into a request against internal schema to store and retrieve data
to and from the database.
The process to convert a request (from external level) and the result between view levels is called mapping. The mapping defines the correspondence between three view levels. The mapping description is also stored in data dictionary. The DBMS is responsible for mapping between these three types of schemas. There are two types of mapping.
(i) External-Conceptual mapping (ii) Conceptual-Internal mapping
External-Conceptual Mapping
An external-conceptual mapping defines the correspondence between a particular external view and the conceptual view. The external-conceptual mapping tells the DBMS which objects on the conceptual level correspond to the objects requested on a particular user's external view. If changes are made to either an external view or conceptual view, then mapping must be changed accordingly.
Conceptual-Internal Mapping
The conceptual-internal mapping defines the correspondence between the conceptual view and the internal view, i.e. database stored on the physical storage device. It describes how conceptual records are stored and retrieved to and from the storage device. This means that conceptual-internal mapping tells the DBMS that how the conceptual! records are physically represented. If the structure of the stored database is changed, then the mapping must be changed accordingly. It is the responsibility of DBA to manage such changes.
