3. What do you mean by logical
data independence?
The ability to change the logical schema without changing the
external schema or application programs is called as Logical
Data Independence.
OR
The ability to change the logical schema without having to
change the external schema.
4. Examples
The addition or removal of new
entities, attributes, or relationships to the
conceptual schema should be possible without
having to change existing external schemas or
having to rewrite existing application programs.
7. What do you mean by Physical Data
Independence
The ability to change the physical schema without
changing the logical schema is called as Physical Data
Independence. Changes in the physical schema may
include.
Using new storage devices.
Using different data structures.
Switching from one access method to another.
Using different file organizations or storage structures.
Modifying indexes.
8. Examples
A change to the internal schema, such as using
different file organization or storage
structures, storage devices, or indexing
strategy, should be possible without having to change
the conceptual or external schemas.
10. Elements of E-R Model
In the E/R model, the structure of data is represented
graphically, as an “entity-relationship diagram,” using
three Principal element types:
Entity Sets.
An entity is an abstract object of some sort, and a
collection of similar entities forms an entity set.
Attributes
The properties of the entities in the set.
Relationship
The connections among two or more entity Sets
11. Example of Elements of E-R Model
Entity Sets
Departments
Professors
Students
Administrators
Attributes
Name of Departments, Phone No., Address...
Name, SSN, Address of Professors...
Relationship
Students and Professors are under a certain department
Admin manage the campus/ departments
12.
13. Attribute
An attribute is an item of data held about an
entity
In computer systems an attribute is a field of
information
Example : Sales System
Entity : Customer
Attributes : Customer Name, Customer
Address, Customer Phone Number
14. A key attribute uniquely identifies a specific occurrence of an
entity
This may also be referred to as a primary key
Examples of primary keys include:
ISBN
Student Number
Staff ID
For Example
In a Sales System, customer name and address are
attributes of the entity customer.
The primary key is likely to be Customer Code or
Customer ID as the unique identifier for each customer in
the system.
15.
16. Example: A library database contains a listing of authors
that have written books on various subjects (one author per
book). It also contains information about libraries that carry
books on various subjects.
Entity sets: authors, subjects, books, libraries
Relationship sets: wrote, carry, indexed
Subject
title isbn matter
SS#
authors wrote books index subject
name
quantity carry
address libraries
