Schema and Instances
Definition of schema:
·
Schema is
the overall design (structural
view (design)) of the entire database.
·
Not
show the data of relation (tables).
·
Not changed frequently.
·
Schema
is three types: Physical schema, logical schema, and view schema.
· For example: In this diagram schema shows the relationship between three tables: Course, Student, and Section.
·
At
physical level ( physical schema):- Define how the data is stored in blocks of
storage.
·
At
logical level (logical schema):- Define types of data records,
stored in data structures.
programmers
and database administrators work at this level.
·
At
view level (view schema):- Define the end-user interaction with database systems.
Advantages
Database Schema
- Manage
data-independent at the physical storage
- Faster
Migration to new GUI.
- Separate
each level.
- It
is more secure.
- The
client doesn't interact directly with the database and business logic.
- No
data loss when one level is fail.
Disadvantages
Database Schema
- Complete
DB Schema is a complex structure.
- Difficult
to understand for everyone.
- Difficult
to set up and maintain.
- physical
Separation of levels can affect the performance of the Database.
=========================================================
DBMS
Instance
Definition of instance:
The data stored in the database at a particular moment of time is called an instance of the database.
Defines the variable declarations at tables of a particular database.
which changes over time when we add or delete data from the database.
For example, in the table student database, the table has 100 records,
so an instance of the database has 100 records.
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