Session 2. Welcome : To the fourth learning sequence “ Entity-Relationship Model “ E-R Model Recap : In the previous learning sequences, we discussed the basic definitions of Data base subject.
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Welcome: To the fourth learning sequence
“ Entity-Relationship Model “
E-R Model
Recap : In the previous learning sequences, we discussed the basic definitions of Data base subject.
Present learning: We shall explore the following definitions:
- Purpose of E-R MODEL.
- E-R Diagram.
- Entity & Attributes.
- Relations.
Entity-Relationship Model1
E-R Diagrams
name
Adviser
Student
name
addr
name
code
Teachers
Courses
teaches
- Teachers are
teaching some
courses.
Number
- Students take
some courses.
Advises
Takes
- Teachers advises
some students.
Students
name
addr
Teachers Courses
Ahmad C++
Samir Math
Ahmad Java
Amer Internet
Samir Algorithms
Jaber Math
Ramez C++
name
addr
name
code
Teachers
Courses
number
Preferences
Students
name
addr
TeacherStudentCourse
AhmadIssam C++
Ahmad NourC++
Ahmad NourJava
Amer IssamInternet
Samir Samar Math
Samir Issam Math
Samir NaderAlgorithms
many-many
* Example of Many-Many Relationship
A customer is associated with several (possibly 0) loans via borrower
A loan is associated with several (possibly 0) customers via borrower
many-one
* Example of Many-One relationship
In a many-to-one relationship a loan is associated with several (including 0) customers via borrower, a customer is associated with at most one loan via borrower
one-one
* Example of One-One relationship
-A customer is associated with at most one loan via the relationship borrower
A loan is associated with at most one customer via borrower
Likes
Students
Courses
Favorite
Takes
Students
Course
Date
Date is a function of both the student and the course, not of one alone.
Relationship Set
HusbandWife
BobAnn
JoeSue
……
Married
husband
wife
Teachers
Summary: In this learning sequence, we discussed the principles of Entity- Relationship diagram.
END