The three-level architecture is also known as three-schema architecture. The purpose of the three-level architecture is to separate the user applications and the physical database. The reasons of this separation are:
(I) Different users need different views of the same data.
(ii) Users should not have to deal directly with the physical database storage details.
(iii) The DBA should be able to change the database storage structure or storage device without affecting other user's views etc.
The three-level architecture is divided into three view levels;
External view level, Conceptual view level and Internal view level.
Internal View Level:
The internal view level is also referred to as physical view level. It describes the physical storage structures of database and file organizations used to store data on physical storage devices. The internal level describes how the data is stored in the database. It works with operating system and DBMS for storing and retrieving data to and from the storage devices. An internal record is a single stored record. It is a storage unit that is passed up to the internal level. The operating system is responsible for storing and retrieving internal records. It means that operating system creates the physical record interface.
The internal view level is described by means of the internal schema, which defines the various record types, storage space allocation for data and indexes etc. The internal schema is written in DDL.
(I) Different users need different views of the same data.
(ii) Users should not have to deal directly with the physical database storage details.
(iii) The DBA should be able to change the database storage structure or storage device without affecting other user's views etc.
The three-level architecture is divided into three view levels;
External view level, Conceptual view level and Internal view level.
Internal View Level:
The internal view level is also referred to as physical view level. It describes the physical storage structures of database and file organizations used to store data on physical storage devices. The internal level describes how the data is stored in the database. It works with operating system and DBMS for storing and retrieving data to and from the storage devices. An internal record is a single stored record. It is a storage unit that is passed up to the internal level. The operating system is responsible for storing and retrieving internal records. It means that operating system creates the physical record interface.
The internal view level is described by means of the internal schema, which defines the various record types, storage space allocation for data and indexes etc. The internal schema is written in DDL.
