Many-to-many with junction tables in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the junction table named 'student_courses'.

SQL

SELECT [1] FROM student_courses;

Drag options to blanks, or click blank then click option'

Acourse_id

Bstudent_id

Cstudents

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column name instead of selecting all columns.

Trying to select from a table that doesn't exist.

The asterisk (*) selects all columns from the table.

2fill in blank

medium

Complete the code to join 'students' and 'student_courses' on the student ID.

SQL

SELECT students.name, student_courses.course_id FROM students JOIN student_courses ON students.[1] = student_courses.student_id;

Drag options to blanks, or click blank then click option'

Aid

Bstudent_id

Ccourse_id

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column name that doesn't exist in 'students'.

Mixing up the foreign key and primary key columns.

The 'students' table's primary key is usually 'id', which matches 'student_id' in the junction table.

3fill in blank

hard

Fix the error in the query to get course names for each student by joining all three tables.

SQL

SELECT students.name, courses.name FROM students JOIN student_courses ON students.id = student_courses.student_id JOIN courses ON courses.[1] = student_courses.course_id;

Drag options to blanks, or click blank then click option'

Astudent_id

Bcourse_id

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the foreign key column name instead of the primary key in the join condition.

Joining on columns that don't relate.

The 'courses' table's primary key is 'id', which matches 'course_id' in the junction table.

4fill in blank

hard

Fill both blanks to create a junction table named 'author_books' with foreign keys to 'authors' and 'books'.

SQL

CREATE TABLE author_books (author_id INT, book_id INT, FOREIGN KEY (author_id) REFERENCES [1](id), FOREIGN KEY (book_id) REFERENCES [2](id));

Drag options to blanks, or click blank then click option'

Aauthors

Bauthor_books

Cbooks

Dbook_authors

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing the junction table itself instead of the related tables.

Mixing up table names.

The junction table references the primary keys of 'authors' and 'books' tables.

5fill in blank

hard

Fill all three blanks to select student names and their course names using joins on the junction table.

SQL

SELECT [1].name, [2].name FROM [3] JOIN student_courses ON students.id = student_courses.student_id JOIN courses ON courses.id = student_courses.course_id;

Drag options to blanks, or click blank then click option'

Astudents

Bcourses

Denrollments

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong table alias or name in the SELECT clause.

Confusing the junction table with main tables.

The query selects from 'students' and 'courses' tables joined through 'student_courses'.

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