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Explain the difference of internal external and conceptual schemas how are these schema layers related to the concept of logical and physical data independence?
Wiki User
∙ 12y ago
External schemas allows data access to be customized (and authorized)
at the level of individual users or groups of users. Conceptual (logical) schemas describes
all the data that is actually stored in the database. While there are several
views for a given database, there is exactly one conceptual schema to all users. Internal
(physical) schemas summarize how the relations described in the conceptual schema
are actually stored on disk (or other physical media).
External schemas provide logical data independence, while conceptual schemas offer
physical data independence.
Wiki User
∙ 12y ago
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Distinguish between logical data independence and physical data independence?
logical data : refer to the immunity of the external scheme towards changes in the conceptual scheme.physical data: refer to the immunity of the conceptual scheme towards changes in the internal scheme.
Mapping in DBMS architecture?
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.
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