DAY - 10
Database
Schema
A database schema is the
skeleton structure that represents the logical view of the entire database. It
defines how the data is organized and how the relations among them are
associated. It formulates all the constraints that are to be applied on the
data.
A database schema defines
its entities and the relationship among them. It contains a descriptive detail
of the database, which can be depicted by means of schema diagrams. It’s the
database designers who design the schema to help programmers understand the
database and make it useful.
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.
Database Instance
It is important that we
distinguish these two terms individually. Database schema is the skeleton of
database. It is designed when the database doesn't exist at all. Once the
database is operational, it is very difficult to make any changes to it. A
database schema does not contain any data or information.
A database instance is a
state of operational database with data at any given time. It contains a
snapshot of the database. Database instances tend to change with time. A DBMS
ensures that its every instance (state) is in a valid state, by diligently
following all the validations, constraints, and conditions that the database
designers have imposed.
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