The ER Model is a planning method used before creating database tables. It helps us identify the real things we want to store, the details about those things, and the connections between them. In common Chen notation, an entity is shown as a rectangle, an attribute as an oval, and a relationship as a diamond. This chapter teaches the complete idea using one college database, memory shortcuts, cardinality, participation, weak entities, relationship attributes and a step-by-step ER diagram method.
What Is the ER Model in DBMS?
ER Model means Entity-Relationship Model. It is a conceptual method used to represent data before building actual database tables. We identify the important objects, their properties, and the relationships between those objects.
Imagine a college wants a database for students, courses and teachers. Before writing SQL, we should answer three simple questions: What things do we need to store? What details do we need about those things? How are those things connected?
Shortcut #1: remember E-A-R
E = Entity · A = Attribute · R = RelationshipIf you can identify these three things from a question, the basic ER diagram is already taking shape.
If table, row, key and SQL still feel new, first revise What Is DBMS? Database Concepts Explained With Real-Life Examples.
Interactive ER Diagram Notation Lab
Click each concept and see the common Chen-style shape. This is the easiest way to stop mixing rectangles, ovals and diamonds.
Shortcut #2: Rectangle = Thing, Oval = Detail, Diamond = Connection
Rectangle → Thing · Oval → Detail · Diamond → ConnectionWhat Is an Entity?
Simple definition
An entity is a real-world object, person, place, event or concept about which we want to store data.
STUDENT
Student ID, name, branch and semester.
COURSE
Course code, title and credits.
TEACHER
Teacher ID, name and department.
DEPARTMENT
Department ID, name and office details.
Entity vs Entity Type vs Entity Set
Shortcut #3: Important nouns are entity candidates
Underline nouns such as Student, Course, Teacher, Department and Book. But do not convert every noun into an entity. “Name” is usually an attribute because it describes another thing.
What Is an Attribute?
Simple definition
An attribute is a property or detail that describes an entity or sometimes a relationship.
For STUDENT, possible attributes are student_id, name, date_of_birth, phone, email, branch and semester.
Shortcut #4: Ask “What do I want to know about this thing?”
Entity = Student. What do I want to know? ID, name, phone, branch. Those answers are attributes.
Types of Attributes in ER Model
Shortcut #5: S-C-S-M-D-K
Simple · Composite · Single · Multi · Derived · KeyMemory sentence: “Simple College Students Make Data Key.” It is intentionally silly so it sticks.
What Is a Relationship?
Simple definition
A relationship shows how two or more entities are connected.
STUDENT ENROLLS COURSE
STUDENT and COURSE are entities. ENROLLS is the relationship.
TEACHER TEACHES COURSE
TEACHER and COURSE are entities. TEACHES connects them.
EMPLOYEE WORKS_FOR DEPARTMENT
WORKS_FOR is the relationship.
STUDENT BORROWS BOOK
BORROWS connects STUDENT and BOOK.
Shortcut #6: Important verbs are relationship candidates
Student ENROLLS · Teacher TEACHES · Customer PLACES OrderDegree of Relationship
Cardinality: 1:1, 1:N, N:1 and M:N
Cardinality tells us how many entities on one side can be associated with how many entities on the other side.
Shortcut #7: Ask two questions
For STUDENT and COURSE: one student can join how many courses? Many. One course can have how many students? Many. Therefore M:N.
This two-question method is much safer than guessing from the sentence.
Total Participation vs Partial Participation
Participation tells whether every entity must take part in a relationship or participation is optional for some entities.
Total Participation
Every entity must participate. In common Chen notation, total participation is shown with a double line.
Partial Participation
Some entities may participate and some may not. It is commonly shown with a single line.
Shortcut #8: “Must” = Total, “May” = Partial
If every entity must participate, think total. If participation is optional, think partial.
Strong Entity vs Weak Entity
Strong Entity
Can be uniquely identified using its own key. Example: EMPLOYEE with employee_id.
Weak Entity
Does not have a complete independent key and depends on an owner entity for identification. Example: DEPENDENT may need employee_id plus dependent_name.
In common Chen notation, a weak entity uses a double rectangle. Its identifying relationship commonly uses a double diamond. A partial key helps distinguish weak entities belonging to the same owner.
Shortcut #9: Weak = cannot stand alone for identification
If the entity needs the owner's key to be identified correctly, think weak entity.
College ER Model Example: Student Enrolls in Course
Requirement: “The college stores students and courses. A student can enroll in many courses, and a course can have many students. For every enrollment, the college stores enrollment date and grade.”
Step 1: Find entities
Nouns give us STUDENT and COURSE.
Step 2: Find relationship
The verb gives us ENROLLS.
Step 3: Find cardinality
One student → many courses. One course → many students. Therefore M:N.
Step 4: Find relationship attributes
enrollment_date and grade describe the enrollment itself, not only the student or only the course.
Possible STUDENT attributes: student_id, name, branch, semester.
Possible COURSE attributes: course_id, title, credits.
Relationship attributes: enrollment_date, grade.
How does this become tables later?
An M:N relationship normally becomes an intermediate relation such as Enrollment(student_id, course_id, enrollment_date, grade). This bridge becomes important when you study relational schema, keys and normalisation.
How to Draw an ER Diagram Step by Step
Shortcut #10: N-D-V-K-C-P-W
Nouns → Details → Verbs → Keys → Cardinality → Participation → Weak checkER Diagram Notation: What Each Symbol Means
The exact visual style can vary between textbooks and tools. This table shows common Chen-style notation.
| Concept | Common Chen Notation | Meaning | Example |
|---|---|---|---|
| Strong Entity | Single rectangle | Independent entity type with its own key. | STUDENT |
| Weak Entity | Double rectangle | Depends on an owner entity for identification. | DEPENDENT |
| Attribute | Oval | Property of an entity or relationship. | Name |
| Key Attribute | Underlined attribute name | Uniquely identifies a strong entity. | student_id |
| Multivalued Attribute | Double oval | Can have multiple values. | Phone Numbers |
| Derived Attribute | Dashed oval | Calculated from other stored data. | Age from DOB |
| Relationship | Diamond | Association between entities. | ENROLLS |
| Identifying Relationship | Double diamond | Connects weak entity to its owner. | EMPLOYEE has DEPENDENT |
| Total Participation | Double line | Every entity must participate. | Every dependent belongs to an employee |
| Partial Participation | Single line | Participation is optional for some entities. | Some teachers manage a department |
What about Crow's Foot notation?
Crow's Foot notation is another popular style, especially in database design tools. It often uses box-like entities and symbols on relationship lines to show minimum and maximum cardinality. The underlying ideas are the same, but the drawing style changes. In exams, follow the notation used by your university or teacher instead of mixing styles randomly.
Minimum and Maximum Cardinality
Sometimes we need to show both whether participation is optional and the maximum number of related entities.
Shortcut #11: First number = minimum, second = maximum
(min, max)If minimum is 0, participation is optional. If minimum is 1, at least one relationship is required.
Entity Attribute vs Relationship Attribute
Entity Attribute
Describes one entity on its own. Example: Student name describes STUDENT.
Relationship Attribute
Describes the connection. Example: grade describes a particular STUDENT-COURSE enrollment.
Shortcut #12: “Does this detail change when the partner changes?”
A student's name does not change from course to course, so it belongs to STUDENT. A student's grade can change for each course, so grade belongs to ENROLLS.
Common ER Diagram Mistakes
How ER Model Connects to Relational Tables
This is why ER modelling should come before detailed table design. The earlier DBMS Architecture chapter explains where database components sit; this chapter explains how to model the information they store.
How to Write ER Model Answers in Semester Exams
Definition + three terms
Define ER Model and write one line each for entity, attribute and relationship.
Add notation
Explain rectangle, oval and diamond with one small example and cardinality.
Explain the complete model
Include attribute types, cardinality, participation, weak entity, relationship degree, notation and one complete ER example.
For presentation technique, use How to Write High-Scoring Answers in University Semester Exams. For revision planning, combine this chapter with the previous-year-paper priority method.
Practice Questions With Answers
1. What is an entity in ER Model?
An entity is a real-world object, person, place, event or concept about which the database stores information. Examples include STUDENT, COURSE and TEACHER.
2. What is the difference between entity and attribute?
An entity is the thing being described, while an attribute is a property of that thing. STUDENT is an entity; name and student_id are attributes.
3. What is a multivalued attribute?
A multivalued attribute can contain more than one value for one entity, such as several phone numbers for one student.
4. What is M:N cardinality?
M:N means many entities on one side can relate to many entities on the other side. STUDENT enrolls COURSE is a common example.
5. What is a weak entity?
A weak entity cannot be completely identified using its own attributes alone and depends on an owner entity for identification.
6. What is the difference between cardinality and participation?
Cardinality tells how many entities may be related. Participation tells whether taking part in the relationship is mandatory or optional.
7. Why is grade an attribute of ENROLLS?
Because a student's grade depends on the particular course. The same student can have different grades in different courses.
8. How can I quickly identify entities and relationships?
Underline important nouns as entity candidates and circle important verbs as relationship candidates, then verify them against the actual requirement.
ER Model One-Minute Revision Sheet
Frequently Asked Questions
What is ER Model in DBMS in simple words?
ER Model is a way to plan a database by identifying the important things, their details and the relationships between them before creating tables.
What are the three main components of ER Model?
Entity, Attribute and Relationship. A simple memory shortcut is E-A-R.
What shapes are used in an ER diagram?
In common Chen notation, entity uses a rectangle, attribute an oval and relationship a diamond. Weak entities and special attributes use double or dashed variations.
What is cardinality in ER Model?
Cardinality tells how many entities on one side can relate to entities on the other side, such as 1:1, 1:N or M:N.
What is a weak entity in DBMS?
A weak entity depends on an owner entity for complete identification because it does not have a sufficient independent key of its own.
What should I study after ER Model?
Study keys and constraints, relational model, ER-to-relational mapping, SQL, functional dependency and normalisation.
What Should You Learn After ER Model?
Recommended sequence
After ER Model, move to Keys in DBMS: super key, candidate key, primary key, alternate key, foreign key and composite key. ER diagrams identify entities and relationships; keys help uniquely identify records and connect tables.
Then continue with constraints, relational model, ER-to-relational mapping, SQL, functional dependency and normalisation. For video revision after reading, use the DBMS YouTube study guide as a secondary resource.
