The DBA ( Database Administrator ) creates the database schema by a set of definitions, that is, translated by the DDL compiler, i.e. stored permanently in the Data dictionary.
The description of a database is called the database Schema, which is specified during database design.
The three-schema architecture is to separate the user applications and the physical database :
- The Internal level has an Internal schema , which describes of the database .The internal schema uses a Physical data model.
- The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of a user. A high-level data model or an implementation data model can be used at this level .
- The External or View level includes a number of External schemas or User views . Each External schema describes the part of database that a particular user group is interested in and hides the rest of the database from the user group .
For example : A schema for a Student and a Course are