DBMS Notes

Three Schema Architecture in DBMS

Three schema architecture hides the details of DB from the user. It’s other name is three Level of Abstraction. The database administrator should be able to change the structure of database as according to need, without affecting the user’s view. This effective three schema architecture holds 3-layers as below

  1. External level (front end developer, user level)
  2. Conceptual level (database designer, logical level)
  3. Internal level (administrator, physical level)

 

1. External or View level

  • This is the highest level of database abstraction.
  • External or view level tells the actual view of data that is relevant to the particular user.
  • View/External level provides different views of the same DB for a specific user or a group of users. For example view for faculty student is different than faculty admin.
  • External level develops with the help of front end programming (Javascript, HTML etc)
  • An external view provides a powerful and flexible security by hiding some information of the database from a particular user.

2. Conceptual or Logical level 

  • The logical/conceptual level tells the structure of the entire database.
  • Conceptual level acts as a intermediate layer between the physical storage(DB) and external level.
  • Database designer works on this layer which provides the structure of database.

  • Database any model (i.e. Relational or ER Model) can use at this level which provides the structure of database.

3. Internal or Physical level 

  • This is the lowest level of database abstraction.
  • Database administrator works on this layer where it has complete control over data
  • It describes how the physical data is actually stored (in the form of file) in the database and provides methods to access data from the database.
  • It shows the physical representation of the database. As it exists actual on the computer system.
  • It means that the changes in physical database storage devices (i.e. hard disks) and the files organization on database storage devices, are hidden to application programs and users.

Note: Data can be stored centralize or distributed in database storage.  At physical level data is stored in the form of file. But it view in the form of table.

Conclusion:

Three level architecture provides the data independence. Because data is independent from user.

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