Schema and Instances

 Schema and Instances 

Definition of schema:

·         Schema is the overall design (structural view (design)) of the entire database.

·         Not show the data of relation (tables).

·         Not changed frequently.

·         Schema is three types: Physical schema, logical schema, and view schema.

·         For example: In this diagram schema shows the relationship between three tables: Course, Student, and Section.


·         At physical level ( physical schema):-  Define how the data is stored in blocks of storage.

·         At logical level (logical schema):- Define types of data records, stored in data structures.

programmers and database administrators work at this level.

·         At view level (view schema):- Define the end-user interaction with database systems.


Advantages Database Schema

  • Manage data-independent at the physical storage
  • Faster Migration to new GUI.
  • Separate each level.
  • It is more secure.
  • The client doesn't interact directly with the database and business logic.
  • No data loss when one level is fail.


Disadvantages Database Schema

  • Complete DB Schema is a complex structure.
  • Difficult to understand for everyone.
  • Difficult to set up and maintain.
  • physical Separation of levels can affect the performance of the Database.




DBMS Instance

Definition of instance:

The data stored in the database at a particular moment of time is called an instance of the database. 

Defines the variable declarations at tables of a particular database. which changes over time when we add or delete data from the database.

For example, in the table student database, the table has 100 records, so an instance of the database has 100 records.

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