Database Schema - The description of the database is called database schema or also metadata. In other words we can say that it contains a descriptive detail of the database, which can be depicted by means of schema diagrams. The database schema is defined during the database design process.It’s the database designers who design the schema to help programmers understand the database and make it useful. Database schema changes very rarely afterwards.
A database schema can be divided broadly into two categories −
Physical Database Schema − This schema pertains to the actual storage of data and its form of storage like files, indices, etc. It defines how the data will be stored in a secondary storage.
Logical Database Schema − This schema defines all the logical constraints that need to be applied on the data stored. It defines tables, views, and integrity constraints.
Meta Data - Metadata is literally "data about data", is information that describes another set of data. A common example is a library catalog card, which contains data about the contents and location of a book: It is data about the data in the book referred to by the card.
Instance - The data in the database at the particular in time is called database state or snapshot.It is also called as current state of occurrences or instances in database.
The term instance in database is also used to describe a complete database environment, including the RDBMS software, table structure, stored procedures and other functionality. It is most commonly used when administrators describe multiple instances of the same database.
Examples: An organization with an employees database might have three different instances: production (used to contain live data), pre-production (used to test new functionality prior to release into production) and development (used by database developers to create new functionality).
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