DBMS is always concerned with some rel world enterprise.Data store in DBMS describe real world entities and represent relationship between these entities. As like there are employees,departments and project in a company and data in the company database describe these entities. In terms of their attribute and relation ship to other entities. Data can be described through different data model and at different levels of abstraction.
1.Data abstraction -: Data abstraction is one of the fundamental characteristic of any database, which help in making data more accurate and easy to use. Abstraction refer to the act of essential feathers without including background details. Data abstraction refers to the act of representing data without giving details that how data are stored or maintained. There are different levels of abstraction
Physical Level -: It is called the internal level. This level is the lowest level of abstraction which specified storage detail that how the data are store in disk.This layer describe manner in which records are stored either as the collection of pages or as the collection of records.
Logical Level -: This is the conceptional view .It is next higher level of abstraction what data are stored in the database and what relation ship exists among those data.There is only one conceptional schema per database. This schema also contain the method of deriving the object on the conceptional view from the internal views. This level of abstraction is used by database administration.
View Level-: View level is the highest level of abstraction which describes different views of the entire database. These views are designed according to the requirement of user who want to access only part of database. In database has many several views,according to the demand of individual user or groups of the user. The data in these views are not exactly store in dbms.
2.Instance 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. Schema are changed infrequently . Database systems have several schema , 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 subschema.