its fundamental characteristic of any dbms. It means to hide unnecessary data from the user, so that only required data is shown to user, there are three level of abstraction.
VIEWS: Physical level or internal level(same) The lowest level of abstraction describes how the data actually stored and data structures and access methods to be used by the database. The internal view is expressed by the internal schema, which contains the definition of the stored record, the method of representing the data fields, and the access aids used.
Logical level or Conceptual view: The next higher level of abstraction describes what data are stored in the database, and what relationship exists among those data. There is only one conceptual schema per database. This schema also contains the method of deriving the objects in the conceptual view from the internal vies. The description of data at this level is in a format independent of its physical representation. It also includes features that specify the checks to retain data consistency and integrity. The logical level of abstraction is used by database administrators, who decide what information is to be kept in the database
View level: The highest level of database abstraction where only those portions of database of concern to a user or application program are included. Any number of user views may exists for a given global or conceptual view. Despite the user of simpler structures at the logical level, some complexity remains, because of the large size of the database. Many users of the database system will not be concerned with all this information. Instead, such users need to access only a part of the database.
INSTANCES AND SCHEMAS: The collection of information stored in the database at a particular moment is called an instance of the database.The overall design of the database is called the database schema. Schemas are changed infrequently. Database systems have several schemas, partitioned according to the levels of abstraction. At the lowest level is the physical schema, at the intermediate level is the logical schema and at the highest level is a subschema. In general database system supports one physical schema, one logical schema, and several subschemas.
DATA INDEPENDENCE: Three levels of abstraction, along with the mappings from internal to conceptual and from conceptual to external; provide two distinct levels of data independence: Logical and physical data independence.
LOGICAL DATA INDEPENDENCE: indicates that conceptual/logical schema can be changed without affecting the existing external (view) schemas. The change would be absorbed by the mapping between the external and conceptual (logical) levels. Consider a change in the conceptual view such as merging two records into one or adding fields to an existing record. This would require a change in the mapping from the external view to the conceptual view so as to leave the external view unchanged. Some changes such as the deletion of a conceptual view field or record, may require changes in the external view and application program. PHYSICAL DATA INDEPENDENCE: indicates that the physical storage structures or devices used for storing the data could be changed without necessitating a change in the conceptual view or any of the external views. The change would be absorbed by the mapping between the conceptual and internal levels. Modifications at physical level are occasionally to improve performance.
