Database Schema in DBMS: Database schema and instances. In today's database management system tutorial, we will learn what is database schema? what are database instances? and what is database state? The description of database is called the “Database Schema”, which is specified during database design and is not expected to change frequently.
Most data models have certain conventions for displaying schemas as diagrams.
A displayed schema is called a “schema diagram.”
Now the problem with schema diagram is that, it does not represent all the aspects of the schema, for example the schema diagram does not represent neither the data types nor the relations of files with each other.
DATABASE STATE AND INSTANCES
The actual data in a database may change quite frequently.
The “data” in a database at a particular moment is called a “database state”.
Database states are also called current set of occurrences or “instances”.
Every time we insert or delete a record or change the value of a data item in a record, we change one state of the database into another.
Database schema v/s Database state
When we define a new database, we specify its database schema only to the DBMS.
When a new database is created, the database state is empty state.
When we populate the database with new initial data, we get the initial database state.
From then on, after every update operation is applied to the database, a new database state is created.
At any given point, the database has a current state.
The dbms is responsible for making sure that every state is a valid state, that is, it satisfies all the constraints and the structure specified in the schema.
DBMS stores the description of schema constraints and the structure of the database which is known as meta-data in catalogue of the dbms, so that the dbms can refer to it, when required.
Although the schema of the database is not changed frequently, its not uncommon for some changes to be applied to the schema when the application requirement change.
This is known as schema evolution.
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