Difference between Data abstraction, schema and instance

Difference between the three levels of data abstraction, and between a schema and instances.. Consider a two dimensional integer array of size n*m that is to be used in your favorite programming language. Using the array as an example, illustrate the difference (a) between the three levels of data abstraction, and (b) between a schema and instances.

Omer Alwi
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Jun 5th, 2011
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Lokesh M
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Nov 19th, 2011

Data Abstraction means suppression of information / details of data organization and storage. Data Abstraction gives importance to mainly essential aspects in order to help better understanding of the database.

A database schema is a description of the database, its structure and its constraints. A database schema does not change with time frequently. Once defined it remains so till the database structure is altered or constraints are added / removed. Schema is also called intension

Instance of the database, also known as the database state, is the actual data available / stored in the database at a particular point of time. Instance is also called extension. The database instance / state changes every time the database is updated.