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Explain Data abstraction or 3 schema architecture.
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Data Abstraction

  • That is the system hides certain implementation details of how data-is stored and Maintained.
  • Reducing Complexity
  • Achieved through three levels of Abstraction-3 Schema Architecture

Schema V/S Instance

Schema

  • Overall design of the database which is specified during db design and will not change frequently.

  • Schema provides names of the entities and attributes and the relationship between these entities.

  • Schema can be represented by using a Schema diagram.

Schema Example

STUDENT

Name Roll no Class Gender

COURSE

CourseName CourseID CourceHrs Dept

Instance

  • The collection of information stored in the database at a particular moment is called instance.
  • Also called database state/snapshot.
  • The actual data in fhe database may change quite frequently.

Instance Example

STUDENT

Name Rollno Class Gender
Akshay 1 Bsc M
Athira P 2 Bsc F
Name Rollno Class Gender
Akshay 1 Bsc M
Athira P 2 Bsc F
Afira 3 Bsc F
Rohit 4 Bsc M

Note

  • Schema does not change frequently but instance changes frequently.
  • However it is possible to change the (schema Schema evolution.

STUDENT

Name Rollno Class Gender DateOfBirth

Three-Schema Architecture

  • DBMS architecture was proposed by ANSI.
  • Its goal is to separate the user application and the physical database.
  • Schemas can be defined at the following three levels
    • External Level
    • Conceptual Level
    • Internal Level

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External Level

  • concerned with the way the data is seen by the individual users.
  • It is closest to the users.

  • Describes the part of the database thata particular user group is interested in.

  • Hides the rĂ©st of the data base from that user group.

Conceptual Level

  • Describes the structure of the whole database.
  • This schema hides the details of physical storage
  • concentrates on describing entities, data types, relationships, user operation and constraints.

Internal Level:

  • It is concerned with the way data is physically stored.
  • It describes the structure of physical storage.

  • It also describes the data structures, file structures and access methods to be used by the database.

Data Independence

  • capacity to change the schema at one level without having to change the schema at the higher level.
  • There are two types of data independence.
    • Logical Data independence
    • Physical Data Independence

Logical Data independence

  • capacity to change conceptual schema without having to change external schemas or application programs.
    • May be to expand the database or to reduce the data base.

Physical Data Independence:

  • Capacity to change internal schema without having to change the conceptual(or external)schemas.
  • Changes to internal schema may be needed because some physical files had to be reorganized to improve performance.
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