Explain the ANSI SPARC architecture

The three-schema architecture is as well known as ANSI SPARC architecture. The aim of the three-schema architecture is to separate the user applications and the physical database. The view at every of these levels is described by a schema. The procedures of transforming requests and results among levels are known as mappings. In this architecture, schemas can be illustrated at the following three levels:

• External Level or Subschema - It is the highest level of database abstraction in which only those portions of the database of concern to a user or application program are involved. Any number of user views might exist.
• Conceptual Level or Conceptual Schema - At this levels of database abstraction all the database entities and the relationships between them are included. There is just only one conceptual schema per database.
• Internal Level or Physical Schema - It is closest to the physical storage method employed.