Many-to-many with junction tables in SQL

Introduction

Many-to-many relationships let us connect items from two groups where each item can link to many from the other group. Junction tables help organize these links clearly.

When students can enroll in many courses, and courses have many students.

When books can have multiple authors, and authors write multiple books.

When customers can buy many products, and products can be bought by many customers.

When movies have many actors, and actors act in many movies.

Syntax

SQL

CREATE TABLE TableA (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);

CREATE TABLE TableB (
  id INT PRIMARY KEY,
  description VARCHAR(100)
);

CREATE TABLE JunctionTable (
  tableA_id INT,
  tableB_id INT,
  PRIMARY KEY (tableA_id, tableB_id),
  FOREIGN KEY (tableA_id) REFERENCES TableA(id),
  FOREIGN KEY (tableB_id) REFERENCES TableB(id)
);

The junction table holds pairs of IDs from the two tables to link them.

Primary key on both columns prevents duplicate links.

Examples

This example shows students and courses linked by enrollments.

SQL

CREATE TABLE Students (
  student_id INT PRIMARY KEY,
  student_name VARCHAR(50)
);

CREATE TABLE Courses (
  course_id INT PRIMARY KEY,
  course_name VARCHAR(50)
);

CREATE TABLE Enrollments (
  student_id INT,
  course_id INT,
  PRIMARY KEY (student_id, course_id),
  FOREIGN KEY (student_id) REFERENCES Students(student_id),
  FOREIGN KEY (course_id) REFERENCES Courses(course_id)
);

This example links authors and books they wrote.

SQL

CREATE TABLE Authors (
  author_id INT PRIMARY KEY,
  author_name VARCHAR(50)
);

CREATE TABLE Books (
  book_id INT PRIMARY KEY,
  book_title VARCHAR(100)
);

CREATE TABLE AuthorBook (
  author_id INT,
  book_id INT,
  PRIMARY KEY (author_id, book_id),
  FOREIGN KEY (author_id) REFERENCES Authors(author_id),
  FOREIGN KEY (book_id) REFERENCES Books(book_id)
);

Sample Program

This query shows which students are enrolled in which courses using the junction table.

SQL

CREATE TABLE Students (
  student_id INT PRIMARY KEY,
  student_name VARCHAR(50)
);

CREATE TABLE Courses (
  course_id INT PRIMARY KEY,
  course_name VARCHAR(50)
);

CREATE TABLE Enrollments (
  student_id INT,
  course_id INT,
  PRIMARY KEY (student_id, course_id),
  FOREIGN KEY (student_id) REFERENCES Students(student_id),
  FOREIGN KEY (course_id) REFERENCES Courses(course_id)
);

INSERT INTO Students VALUES (1, 'Alice'), (2, 'Bob');
INSERT INTO Courses VALUES (101, 'Math'), (102, 'History');
INSERT INTO Enrollments VALUES (1, 101), (1, 102), (2, 101);

SELECT s.student_name, c.course_name
FROM Enrollments e
JOIN Students s ON e.student_id = s.student_id
JOIN Courses c ON e.course_id = c.course_id
ORDER BY s.student_name, c.course_name;

Important Notes

Always use foreign keys in the junction table to keep data linked correctly.

Composite primary key in the junction table avoids duplicate pairs.

You can add extra columns in the junction table for details about the relationship, like enrollment date.

Summary

Many-to-many relationships connect items from two tables in pairs.

Junction tables store these pairs with foreign keys to both tables.

This setup keeps data organized and easy to query.

Practice

1. What is the main purpose of a junction table in a many-to-many relationship?

easy

A. To store pairs of related records from two tables using foreign keys

B. To store all data from both tables in one place

C. To replace one of the original tables completely

D. To create a one-to-one relationship between tables

Solution

1. Step 1: Understand many-to-many relationships

   Many-to-many means each record in one table can relate to many records in another table, and vice versa.

2. Step 2: Role of junction table

   A junction table holds pairs of foreign keys from both tables to link related records without duplicating data.

3. Final Answer:

   To store pairs of related records from two tables using foreign keys -> Option A

4. Quick Check:

   Junction table = pairs of foreign keys [OK]

Hint: Junction tables link two tables with pairs of keys [OK]

Common Mistakes:

• Thinking junction table stores all data from both tables
• Confusing junction table with a single main table
• Assuming junction table creates one-to-one links

2. Which SQL statement correctly creates a junction table named StudentCourse linking Student and Course tables by their IDs?

easy

A. CREATE TABLE StudentCourse (StudentID INT, CourseID INT, FOREIGN KEY (StudentID) REFERENCES Student(ID));

B. CREATE TABLE StudentCourse (ID INT PRIMARY KEY, StudentID INT, CourseID INT);

C. CREATE TABLE StudentCourse (StudentID INT UNIQUE, CourseID INT UNIQUE);

D. CREATE TABLE StudentCourse (StudentID INT, CourseID INT, PRIMARY KEY (StudentID, CourseID));

Solution

1. Step 1: Define junction table columns

   It needs two columns for foreign keys: StudentID and CourseID.

2. Step 2: Set primary key on both columns

   Primary key on (StudentID, CourseID) ensures unique pairs and no duplicates.

3. Final Answer:

   CREATE TABLE StudentCourse (StudentID INT, CourseID INT, PRIMARY KEY (StudentID, CourseID)); -> Option D

4. Quick Check:

   Junction table needs composite primary key [OK]

Hint: Use composite primary key on both foreign keys [OK]

Common Mistakes:

• Using UNIQUE on individual columns instead of composite key
• Missing one foreign key column
• Not defining primary key on the pair

3. Given tables Author, Book, and junction table AuthorBook with columns AuthorID and BookID, what does this query return?

SELECT Author.Name, Book.Title FROM Author
JOIN AuthorBook ON Author.ID = AuthorBook.AuthorID
JOIN Book ON Book.ID = AuthorBook.BookID;

medium

A. A list of authors and the titles of books they wrote

B. A list of books without any author names

C. A list of all authors with all books, including unrelated pairs

D. An error because of missing WHERE clause

Solution

1. Step 1: Understand JOINs with junction table

   The query joins Author to AuthorBook by AuthorID, then AuthorBook to Book by BookID, linking authors to their books.

2. Step 2: Result of the query

   It returns pairs of author names and book titles where the author wrote the book, no unrelated pairs included.

3. Final Answer:

   A list of authors and the titles of books they wrote -> Option A

4. Quick Check:

   JOINs with junction table = related pairs only [OK]

Hint: JOIN junction table to get related pairs only [OK]

Common Mistakes:

• Thinking it returns all combinations of authors and books
• Expecting an error without WHERE clause
• Ignoring the role of junction table in filtering

4. You wrote this query to find all students and their courses:

SELECT Student.Name, Course.Title FROM Student
JOIN StudentCourse ON Student.ID = StudentCourse.StudentID
JOIN Course ON Course.ID = StudentCourse.CourseID
WHERE StudentCourse.StudentID = Student.ID;

But it has a problem. What is the problem?

medium

A. The WHERE clause is redundant and causes a syntax error

B. Missing alias for tables causes ambiguity

C. The WHERE clause is unnecessary because JOIN already matches IDs

D. StudentCourse.CourseID is missing in the WHERE clause

Solution

1. Step 1: Analyze the JOIN conditions

   The JOINs already match Student.ID to StudentCourse.StudentID and Course.ID to StudentCourse.CourseID.

2. Step 2: Check the WHERE clause

   The WHERE clause repeats the JOIN condition, which is unnecessary but not an error; however, it does not filter or add value.

3. Final Answer:

   The WHERE clause is unnecessary because JOIN already matches IDs -> Option C

4. Quick Check:

   JOIN matches IDs, WHERE clause redundant [OK]

Hint: JOIN conditions handle matching; WHERE often not needed here [OK]

Common Mistakes:

• Assuming WHERE clause causes syntax error
• Adding unnecessary conditions that duplicate JOINs
• Confusing alias usage with errors

5. You have tables Employee, Project, and junction table EmployeeProject with EmployeeID and ProjectID. How do you find employees who work on all projects listed in Project?

hard

A. Join EmployeeProject and Project, then filter with WHERE ProjectID IS NOT NULL

B. Use GROUP BY EmployeeID and HAVING count of projects equal to total projects count

C. Select employees with a simple JOIN to EmployeeProject without grouping

D. Use DISTINCT on EmployeeID in EmployeeProject without counting projects

Solution

1. Step 1: Count total projects

   Find total number of projects from Project table.

2. Step 2: Group EmployeeProject by EmployeeID

   Count how many projects each employee works on.

3. Step 3: Use HAVING to compare counts

   Only select employees whose project count equals total projects count.

4. Final Answer:

   Use GROUP BY EmployeeID and HAVING count of projects equal to total projects count -> Option B

5. Quick Check:

   Group and count projects per employee = all projects [OK]

Hint: Group by employee, HAVING count = total projects [OK]

Common Mistakes:

• Not grouping and counting projects per employee
• Using WHERE instead of HAVING for aggregate filtering
• Ignoring total projects count in comparison