Database Management Systems (dbms)

Instance

Data (the actual content) in the database at a particular moment in time is called database state.

It is also called the instances or current set of occurrences

Three Schema Architecture

External Level
Conceptual Level
Internal Level
External / Conceptual mapping
External
View 1
External
View n
Conceptual
Schema
Internal
Schema
DB
DB
DB
Conceptual / Internal mapping
End users

Three Schema Architecture..

Internal (Physical) Schema

• Defines the physical view of data (as seen by a DBMS)
• Describe the physical storage of the database
• Describe how the data is stored in the database
• Concern with storage space allocation, record description, data compression and encryption techniques.

Internal (Physical) Level

• Struct Employee {

Char Emp_no[5],

Char Name[25],

Date Date_joined,

Boolean Pay_Tax,

Int OT_Hours,

Char Des_Code[3]

} ;

Three Schema Architecture..

Conceptual Schema

• Defines the logical view of data (as seen by programs)
• Describe the structure of the whole database for a community of users.
• Hides the details of physical storage structure.
• Describe entities, data types, relationships, user operations and constraints.

Conceptual Level

Employee Table
Base Table

Three Schema Architecture..

External Schema

• Defines the external view of data (as seen by a user)
• The users view of the database
• Describe the part of the database that a particular user group is interested.
• Hides the other details of the database.