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Schema and Instances in Database Management System

In this article, we are going to discuss about introduction of schema, types of schema, Instances, some basics about the data independence in database management system. Submitted by Prerana Jain, on May 30, 2018

Schema in Database Management System

The overall design of a database is called database Schema. A schema is a plan of the database that gives the names of the entities and attribute and the relationship among them. A scheme includes the definition of the database names, the record type and the components that make up the records. Alternatively, it is defined as a framework into which the values of the data items are fitted. The values in the schema changes but the format of schema does not change.

Example:

Consider the database consisting of three files ITEM, CUSTOMER, and SALES. The structure design for this schema is shown...

Type	ITEM = record
    ITEM_ID: string;
    ITEM_DESC: string;
    ITEM_COST: integer;
    End	

There are mainly three types of schema

1. Physical Schema

It is database design at the physical level. It is hidden below logical schema and can be designed easily without affecting application programs. The physical schema is concerned with the manner in which the conceptual database get represented in the computer as a stored database. The physical schema is hidden and may be changed easily. The DBMS’s provide DDL and DSDL to specify both logical and physical schema.

2. Logical Schema

It is database design at the logical level. Programmer construct applications using logical schema. It is by far the most important schema in terms of its effect on the application program. The logical schema exploits the data structure which is offered by the DBMS that’s why the schema is understandable for the computer system.

3. Subschema

It is schema at the view level. Subschema is a subset of having the same properties that a schema. It identifies a subset of cities, sets, record, and data names defined in the database schema which is available for the users. The subschema allows the user to view only the part of the database that is of interest to him. The different application programs can change their respective subschema without affecting other’s schema or view.

Instances in Database Management System

The Instances are the data which is stored in the database at any particular moment in time. In a given instances each schema construct has its own current set of instances. Many instances or database states can be constructed to correspond to a particular database schema. Every time we update means update, delete or modify the values of a data item in a record one state of the database changes into another state.