Instances and Schemas

Databases change over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called an instance of the database. The overall design of the database is called the database schema.

Consider the following database table:

Employee Table:
 

In the Employee Table, schema and instance are clearly shown.

Database systems have several schemas, partitioned according to the levels of abstraction. The physical schema describes the database design at the physical level, while the logical schema describes the database design at the logical level. A database may also have several schemas at the view level, sometimes called subschemas, that describe different views of the database.

The logical schema is the most important among all the schemas since programmers construct application programs or front end interfaces by using logical schemas. The physical schema is hidden beneath the logical schema, and can usually be changed easily without affecting logical level. Hence application programs do not need to be rewritten if physical schema changes and are said to exhibit physical data independence.

I am writing a book on Databases. In the current blog, I will give you every Friday morning BDT time, small excerpts on Databases from my book in order to brush up on the subject. My eBook on Databases is a copyright in 2015 by me, Rosina S Khan. No part(s) or whole of these excerpts may be used or reproduced in any form whatsoever, without written permission by the author.