Draw and explain the three level architecture of the database system?

A DBMS gives three levels of data is said to follow three-level architecture. The trget of the three-schema architecture is to divide the user applications and the physical database. The view at each of these stages is described through a schema. The processes of transforming requests and results among levels are known as mappings. In this architecture, schemas can be described at the subsequent three stages:

External Level or Subschema - It is the highest level of database abstraction whereas only those portions of the database of concern to a user or application program are involved. Any number of user views (some of which may be identical) might exist for a given global or conceptual view. Each external view is described through means of a schema known as an external schema or subschema.

Conceptual Level or Conceptual Schema - At these stages of database abstraction all the database entities and the relationships between them are included. One conceptual view represents the whole database. This conceptual view is described through the conceptual schema. There is only one conceptual schema per database. The description of data at that level is in a format independent of its physical representation. It also involves features which specify the checks to retain data consistency and integrity.

Internal Level or Physical Schema - It is closest to the physical storage technique used. It denotes how the data will be stored and elaborates the data structures and access techniques to be used through the database. The internal view is expressed through the internal schema.