- 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
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.
- 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.
- Schema does not change frequently but instance changes frequently.
- However it is possible to change the (schema Schema evolution.
- 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
- External Level
- Conceptual Level
- Internal 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.
- 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.
- 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.
- 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
- 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.