Schema in DBMS and its types

Three Schema Architecture of DBMS:

Three degrees of abstraction are used to explain the data in a DBMS.
At each of these three levels of abstraction, there is a schema that makes up the database description.
- External Schema
- Conceptual Schema
- Physical Schema

External schemas enable data access to be configured (and approved) at the level of individual users or groups of users. These schemas are often also in terms of the DBMS’s data model.
There are several external schemas in any particular database, but only one conceptual and one physical schema.

For the benefit of the user community, the conceptual schema details the whole database’s structure.
This schema concentrates on defining data types, entities, relationships, etc. while concealing details about the storage structures.

Further storage information is specified in the physical schema.
The real way that the relations are kept on secondary storage media like disks and tapes is summed up in this schema.
To expedite data retrieval operations, this schema establishes which file organizations to utilize for storing the relations and generates supplemental data structures known as indexes.

Using a database management system (DBMS) has several benefits, one of which is data independence, which protects application applications from modifications to the structure and storage of the data.
Using the three layers of data abstraction leads to data independence:
Logical Data Independence: The ability to modify the conceptual framework without requiring modifications to the external frameworks and their corresponding application programs.
Physical Data Independence: The ability to change the internal structure without affecting the overall concept.

If a schema is modified at a lower level,
- In a database management system that fully supports data independence, the only mappings that need to be modified are those between this schema and higher-level schemas.
- Those top-level schemas don’t change at all.
- Because the application programs refer to the external schemas, they do not require modification.

A schema in DBMS defines the structure and organization of data within a database.
It acts as a blueprint, outlining the entities, their attributes, relationships, and constraints. Schemas provide a framework for understanding and manipulating data, ensuring consistency and integrity.

The three-schema architecture defines three levels of abstraction for a database:
The external schema represents the user’s view of the data, the conceptual schema defines the overall data structure and relationships, and the internal schema details the physical storage and implementation. This architecture enables data independence, allowing changes at one level without affecting others.

Data independence allows changes to the database’s structure without affecting how applications or users see the data.
This separation allows for flexible database design and implementation without disrupting existing applications.

The three-schema architecture facilitates data independence by separating the different levels of abstraction.
The external schema is protected from changes to the internal or conceptual schema, ensuring that applications remain stable and users experience consistent access.

Data independence offers numerous benefits, including flexibility in database design and implementation, reduced impact of changes on applications, ease of database maintenance, and improved system scalability.

An example of physical data independence is changing the storage format of a table without affecting the user’s view.
An example of logical data independence is adding a new attribute to a table without impacting applications using the existing schema.