9.4 Cardinality (1:1, 1:N, M:N)

Level: 9 | Version: v1.1 | Author: Meptrasoft

Overview

Cardinality describes how many records in one table can be related to records in another table.

The three common types are:

Type	Meaning	Example
1:1	One ↔ One	Person ↔ Passport
1:N	One ↔ Many	Department ↔ Employees
M:N	Many ↔ Many	Students ↔ Courses

Database Cardinality: 1:1, 1:N, M:N relationships — Figure 1: Cardinality describes how many records can be related between entities: one-to-one, one-to-many, or many-to-many.

1:1 — One-to-One

One record in Table A is related to one record in Table B.

Example:

PERSON 1 ─── 1 PASSPORT

A person has one passport, and that passport belongs to one person.

1:1 → One to One

1:N — One-to-Many

One record in Table A can relate to many records in Table B.

Example:

DEPARTMENT 1 ───< EMPLOYEES

One department can have many employees.

1:N → One to Many

M:N — Many-to-Many

Many records in Table A can relate to many records in Table B.

Example:

STUDENTS >───< COURSES

A student can take many courses, and a course can have many students.

In a relational database, this is normally implemented using a junction (bridge) table:

Students
   ↓
Enrollment
   ↓
Courses

M:N Example in SQL

CREATE TABLE enrollment (
    student_id INT REFERENCES students(student_id),
    course_id INT REFERENCES courses(course_id)
);

The enrollment table connects students and courses.

A query can retrieve the relationship:

SELECT s.student_name, c.course_name
FROM students s
JOIN enrollment e
    ON s.student_id = e.student_id
JOIN courses c
    ON e.course_id = c.course_id;

Quick Comparison

1:1 → ONE ↔ ONE
1:N → ONE ↔ MANY
M:N → MANY ↔ MANY

M:N relationships are usually implemented using a junction table.

Common Beginner Mistakes

✗ Mistake	✓ Correct Understanding
1:N means one table has many rows	It describes the relationship between records/entities
M:N is directly stored in one foreign-key column	It is normally implemented using a junction table
Cardinality and foreign key are the same	Cardinality describes the relationship; a foreign key helps implement it
One student can have only one course in an M:N relationship	A student can have many courses

Placement Quick Points

1:1 → One record relates to one record.
1:N → One record relates to many records.
M:N → Many records relate to many records.
M:N relationships are commonly implemented using a junction table.
Cardinality helps define database relationships.

Interview Questions

What is cardinality?

Cardinality describes how many records can participate in a relationship between entities.

What is 1:1?: One record relates to exactly one record.
What is 1:N?: One record can relate to many records.
What is M:N?: Many records can relate to many records.
How is M:N implemented in a relational database?: Using a junction/bridge table containing foreign keys referencing the two related tables.

Practice & Hands-On Exercises

Give one real-world example of 1:1.
Give one example of 1:N.
Give one example of M:N.
Explain why a junction table is needed for M:N.
Identify the cardinality between Department and Employee.

💡 Tip: Test your queries and schema definitions using the in-browser interactive runner above.

Key Takeaway

1:1 → ONE TO ONE
1:N → ONE TO MANY
M:N → MANY TO MANY

Cardinality describes how many records can be related, with M:N relationships commonly implemented through a junction table.

End of Cardinality (1:1, 1:N, M:N)