January 30, 2020
Amitraj
Instance -
The data stored in database at a particular moment of time is called instance of database.
Database schema defines the variable declarations in tables that belong to a particular database; the value of these variables at a moment of time is called the instance of that database.
for example, lets say we have a single table student in the database, today the table has 100 records, so today the instance of the database has 100 records. Lets say we are going to add another 100 records in this table by tomorrow so the instance of database tomorrow will have 200 records in table. In short, at a particular moment the data stored in database is called the instance, that changes over time when we add or delete data from the database.
Schema -
Design of a database is called the schema. Schema is of three types: Physical schema, logical schema and view schema.
-> A relation schema describes the relation name (table name), attributes, and their names.
1. A physical schema can be defined as the design of a database at its physical level. In this level, it is expressed how data is stored in blocks of storage.
2. Design of database at logical level is called logical schema, programmers and database administrators work at this level, at this level data can be described as certain types of data records gets stored in data structures, however the internal details such as implementation of data structure is hidden at this level (available at physical level).
3. Design of database at view level is called view schema. This generally describes end user interaction with database systems.
