Mapping:
The process to convert a request from external level and result between view levels is called mapping. It defines the correspondence between three view levels; the mapping description is also stored in data dictionary.
In detail, three view levels are described by means of three schemas. Schemas are stored in data dictionary. In DBMS, each user refers only to its own external schemas. 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. DBMS is responsible for mapping between these three types of schemas.
There are two types of mapping:
1. External conceptual mapping
2. Conceptual physical mapping
1. External conceptual mapping:
It defines the correspondence between a particular external view and the conceptual view. It shows that which object on the conceptual level corresponds to the object requested on a particular user’s external view. In case of any changes between the views the mapping must be changed accordingly.
2. Conceptual physical mapping:
It defines the correspondence between the conceptual view and physical view i.e. database stored in physical storage device. It shows how conceptual records are stored and retrieved to and from the storage device. Overall, it describes that how conceptual records are physically stored. If the structure of stored database changed then mapping must also be changed accordingly
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