Mapping In Database
Mapping In Database. The process to convert a request and the result from one level to another level is known as Mapping. The mapping defines the correspondence between three levels. The mapping description is also stored in data dictionary in the same file of data. The DBMS is responsible for mapping between these three types of schemas in the database of the same file to represent it. There are two types of mapping
- External-Conceptual Mapping
- Conceptual-Internal Mapping
An external-conceptual mapping defines the correspondence between a particular external view and conceptual view for representing the data in efficient manner. 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 for information representation in the same data file. If changes are made to either an external view or conceptual view, then mapping must be changed accordingly in the data file of the same organization. It is an important mapping in database
The conceptual-internal mapping defines the correspondence between the conceptual view and internal view, i.e. database stored on the physical storage device. It describes how conceptual records are stored and retrieved to and from storage device. It means that conceptual-internal mapping tells the DBMS that how the conceptual record are physically represented. If the structure of the stored database is changed, then the mapping must be changed accordingly and must be saved accordingly to the data file. It is the responsibility of DBA to manage such changes and control it accordingly to save data from damage.
The DBMS receive the request and checks the user’s external schema, external-conceptual mapping and conceptual schema in data dictionary. The DBMS also checks the user’s authorization to access the record. If the user is not recognized then his/her request is rejected. If the user is authorized, then his request is passed to the conceptual level. In checks the conceptual-internal mapping to see corresponding internal structure of mapping in database. DBMS identifies the internal objects or items that are required to the operating system on physical level.