What do you mean by logical data
The ability to change the logical schema without changing the
external schema or application programs is called as Logical
The ability to change the logical schema without having to
change the external schema.
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.
What do you mean by Physical Data
The ability to change the physical schema without
changing the logical schema is called as Physical Data
Independence. Changes in the physical schema may
Using new storage devices.
Using different data structures.
Switching from one access method to another.
Using different file organizations or storage structures.
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
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:
An entity is an abstract object of some sort, and a
collection of similar entities forms an entity set.
The properties of the entities in the set.
The connections among two or more entity Sets
Example of Elements of E-R Model
Name of Departments, Phone No., Address...
Name, SSN, Address of Professors...
Students and Professors are under a certain department
Admin manage the campus/ departments
An attribute is an item of data held about an
In computer systems an attribute is a field of
Example : Sales System
Entity : Customer
Attributes : Customer Name, Customer Address,
Customer Phone Number
A key attribute uniquely identifies a specific occurrence of an
This may also be referred to as a primary key
Examples of primary keys include:
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
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
books indexwrote subjectauthors
Entity-Relationship (E-R) Model
COURSE# CTITILE PROF SCHED
STUID SNAME MAJOR CREDITS
Example of the 3 elements in Entity