COMP231 Tutorial 1COMP231 Tutorial 1

ER Model and ER to Relational Schema

Fall 20062Database Management Systems

E-R DiagramE-R Diagram

Rectangles – entity sets

Ellipses – attributes

Diamonds – relationship sets

Double ellipses – multivalued attributes

Dashed ellipses – derived attributes

Double lines – total participation

Double rectangles – weak entity sets

Double diamonds – identifying relationship sets

Fall 20063Database Management Systems

Exercise 1.1 Construct E-R DiagramExercise 1.1 Construct E-R Diagram

A university registrar’s office maintains data about the following entities:

courses, including number, title, credits, syllabus, and prerequisites;

course offerings, including course number, year, semester, section number, instructor(s), timings, and classroom;

students, including student-id, name, and program;

instructors, including identification number, name, department, and title.

Further, the enrollment of students in courses and grades awarded to students in each course they are enrolled must be appropriately modeled.

Construct an E-R diagram for the registrar’s office.

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EntitiesEntities

course

course-offeringstudent instructor

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CourseCourse

“ courses including number, title, credits, syllabus, and prerequisites”

Attribute?

Relationship?

course

courseno

title

credits

syllabusrequires

prerequisite

maincourse

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Course OfferingCourse Offering

“ course offerings, including course number, year, semester, section number, instructor(s), timings, and classroom”

course- offering

year

semestertime

room

secno

courseis_offered

courseno

Fall 20067Database Management Systems

Weak EntityWeak Entity

A weak entity can only be identified uniquely by combining the primary key of another (owner) entity and the partial key of itself.

Owner entity set and weak entity set must participate in one-to-many relationship (one owner entity, many weak entities).

Weak entity set must have total participation in this identifying relationship set.

Fall 20068Database Management Systems

Student, InstructorStudent, Instructor

“ students, including student-id, name, and program”

“instructors, including identification number, name, department, and title”

student

namesid

program

instructor

titledept

nameiid

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EnrollmentEnrollment

“ Further, the enrollment of students in courses and grades awarded to students in each course they are enrolled must be appropriately modeled.”

student course-offeringenrolls

grade

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Anymore??Anymore??

Instructor teaches course…..

course-offering

instructorteache

s

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E-R Diagram for a UniversityE-R Diagram for a University

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Exercise 1.2 Covert E-R Diagram into TablesExercise 1.2 Covert E-R Diagram into Tables

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Entities (Not Weak)Entities (Not Weak)

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Entities (Not Weak)Entities (Not Weak)

course (courseno, title, syllabus, credits)

student (sid, name, program)

instructor (iid, name, dept, title)

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Weak EntitiesWeak Entities

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Weak EntitiesWeak Entities

course-offering (courseno, secno, year, semester, time, room)

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Relationships (Not defining weak entities)Relationships (Not defining weak entities)

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Relationships (Not defining weak entities)Relationships (Not defining weak entities)

enrolls (sid, courseno, secno, semester, year, grade)

teaches (courseno, secno, semester, year, iid)

requires (maincourse, prerequisite)

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Relationships with Weak EntitiesRelationships with Weak Entities

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Relationships with Weak EntitiesRelationships with Weak Entities

There is no extra table for the relationship between a weak entity and its strong entity.

The relationship is already present in the schema for the weak entity.

course-offering (courseno, secno, year, semester, time, room)

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Relational Schemas for a UniversityRelational Schemas for a University

course (courseno, title, syllabus, credits)

student (sid, name, program)

instructor (iid, name, dept, title)

course-offering (courseno, secno, year, semester, time, room)

enrolls (sid, courseno, secno, semester, year, grade)

teaches (courseno, secno, semester, year, iid)

requires (maincourse, prerequisite)

Fall 200622Database Management Systems

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