Database Schema in RDBMS

It is the structure of database. Schema defines how the data will be stored in the database, relationship between entities and attributes and so on.  

The overall structure or the overall design of a database is the database schema. The term “schema” refers to the organization of data as a blueprint of how the database is constructed. The schema is a complete description of a database.

A schema contains schema objects, which could be tables, columns, data types, views, stored procedures, relationships, primary keys, foreign keys, etc. It describes the overall database.

Schema is of three types:

     Physical Schema/Internal Schema: It describes the database design at the physical level. The physical schema defines how database is stored in secondary storage.

 

     Logical Schema/Conceptual Schema: It describes the database design at the logical level. This schema defines logical structure of a database.

 

     View Schema/External Schema: It defines the design of the database at the view level.

 

 

Subschema:

It is the subset or sub part of the schema having the same properties that a schema has. It identifies a subset of areas, sets, records and data names defined in the database schema

Subschema describes different view of the database. A subschema lets the user have access to different areas of application in which the user designed. Schemas may have many different sub schemas that are all very different.

 

For example: Suppose a table student, programmer has access of all columns of table student but user has access of only two or three columns of table student.