A major of purpose of a database system is to provide users with an abstract view of the data. that is the system hides certain details of how the data are Stored and maintained. since many database systems users are not computer trained, developers hide the complexity from uses through several level of abstraction, to simplify users interaction with the system. this is known as data abstraction.
There are 3 level of data abstraction.
1. Physical level – the lowest level of abstraction describes how the data actually stored.
2. Logical level– the next higher level of abstraction describes What data are stored in the database, and What relationship exist among those data.
3. View level– the highest level of abstraction describes only part of the entire database. the system may provide many views for the same database.
Please check the Figure below
the overall design of the database is called the database schema. another word, schema means n overall plan of all the data item types and record types stored in the database. schema includes definition of the database name, record type and the component that make up those records.
database may also have several schema at the view level, sometimes called sub schemas, that describe different view of the database.
the ability to modify a schema definition in 1 level without affecting schema definition in a higher level is called data independence
- Physical data independence:application programs are said to to exhibit physical data independence if they do not depend on the physical schema, and thus need not be rewritten if a physical schema changes.
- Logical data independence:the ability to modify the conceptual schema without causing application program to be rewritten.
when the schema framework is filled in the data item values or the content of the database at any point of time, it is referred to as an instance update database. the term instance is also called as state of the database or snapshot. each variable has a particular value at a given instance. the value of the variables any program at a point in time corresponds to an instance of a database schema.