A schema is the structure that represents the logical view of databases. It defines how data is organized and how the relations among them are associated. It formulates all the condition that are to be applied on data. Data independence is the characteristic of being able to modify the schema at one level of the database system without disturbing the schema at the next higher level.
THREE SCHEMA ARCHITECTURE:
The three level of schema are:
- external level or view level
- conceptual level or logical level
- internal level or physical schema
At the external level, a database contains several schemas that sometimes called as subschemas. The subschemas is use to describe the different views of database. Each external schema describes the part of the database that a particular user group is interested in and hides the rest of the database from that group
CONCEPTUAL OR LOGICAL LEVEL
The conceptual schemas describe the design of database at the conceptual level. It describes the structure of whole database and what should be store in the database. This level defines all database entities ,their attributes, and their relationships.
INTERNAL OR PHYSICAL LEVEL
The internal level has an internal schema which describes the physical structure of the database. The internal schema uses a physical data model and describes the complete details of data storage and access path for the database.
- LOGICAL DATA INDEPENDENCE (at conceptual level)
- PHYSICAL DATA INDEPENDENCE (at physical level)
LOGICAL DATA INDEPENDENCE:
It is the characteristic of being able to modify the conceptual schema or changes in the middle level of DBMS without affecting the next higher level . It occurs at the user interface level.
PHYSICAL DATA INDEPENDENCE:
It is the characteristic of changing the internal schema without having to change conceptual schema. With physical data independence we can easily change the physical storage structures or devices. It occurs at the logical interface level.