1. Department of Information Technology
Jagannath International Management School
Vasant Kunj, New Delhi - 110070
(Affiliated to Guru Gobind Singh Indraprastha University, New Delhi)
Recognized u/s 2(f) by UGC & Accredited with ‘A’ Grade by NAAC
Participant of UNGC & UNPRME, New York
ISO 9001:2015 Quality Certified
Subject Name : Database Management Systems (DBMS)
Subject Code: BCA-108
Created By: Dr. Meenakshi Narula
4. Schemas, Instances and Database State
Database Schema (meta-data): The Design of a database is called the
schema. It Includes descriptions of the database structure and the
constraints that should hold on the database. The database schema
changes very infrequently.
Database Instance: The actual data stored in a database at a particular
moment in time. Also called database state ( or occurrence, snapshot)
The database state changes every time the database is updated.
Schema is also called intension, whereas state is called extension.
7. DBMS Architecture
External schema at the external level to describe the various user views. Usually
uses the same data model as the conceptual level or high-level data model.
Conceptual schema at the conceptual level to describe the structure and constraints
for the whole database. Uses a conceptual or an implementation data model.
Internal schema at the internal level to describe data storage structures and access
paths. Typically uses a physical data model.
Three-Schema Architecture
9. The External or View Level Schema
Each end users’ view of the data environment
Modeler subdivides requirements and constraints into
functional (Business unit’s) modules
10. External Schema – Advantages
• Easy to identify specific data
• Facilitates designer’s job
• Creation of external models
• Simplifies application program development
11. The Conceptual or Logical Level Schema (1 of 2)
• Global view of the entire database
• Representation of data for the entire organization
12. The Conceptual Level Schema (2 of 2)
• Software and hardware
independent
• Most widely used conceptual
model is the Entity Relationship
(ER) model
13. The Internal or Physical Level Schema
• The database as “seen”
by the DBMS
• Maps the conceptual
model to the DBMS
• Depicts a specific
representation of an
internal model
• Logical independence
14. Three Schema Architecture – Advantages
• Database abstraction
• Easier to use for a user.
• Allows each user to access customized view of data.
• Enables a database admin to change the storage structure
without affecting the user’s view
15. Data Independence
The capacity to change the schema at one level without having to change the schema
at the next higher level
Types:
Logical Data Independence: The capacity to change the conceptual schema without
having to change the external schemas and their application programs.
Physical Data Independence: The capacity to change the internal schema without
having to change the conceptual schema.
Requires only the mappings between one schema and higher-lever schemas to
change
