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

COURSE

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

Note

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

STUDENT

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

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 rést 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.