Schemas and Instances

SCHEMA

The logical structure of the database is known as a logical schema. For example, The database consists of a set of customers' information, accounts' information and the relationship between them. Schema is analogous to type information of a variable in a program. There are three types of schema: Physical schema, Logical schema, and view schema.

1. Physical Schema: The database designed at the physical level is known physical schema. This level explains how the data is stored in blocks of storage.
2. Logical Schema: The database designed at logical level is the known logical schema.The programmers, as well as the database administrators, work at this level. The data can be described as certain types of data records gets stored in data structures, however, the internal details such as the implementation of the data structure are hidden at this level which is available at the physical level.
3. View Schema: The database designed at view level is called view schema. This level generally describes end user interaction with database systems.

INSTANCE

The actual content of the database at a particular point in time is known as Instance. The database schema defines the variable declarations in tables that belong to a particular database and the value of these variables at a moment of time is called the instance of that database.

References:

1. H.F.Korth and A. Silberschatz,"Database system concepts",McGraw Hill,2010
2. A.K.Majumdar and p, Bhattacharya,"Database Management Systems",Tata McGraw Hill,India,2004
3. F.Korth, Henry. Database System Concepts. 6th edition.