One-to-many relationship design in SQL - Mini Project: Build & Apply

One-to-many relationship design

📖 Scenario: You are building a simple database for a library. Each author can write many books, but each book has only one author. You want to organize this data so you can easily find all books by a specific author.

🎯 Goal: Create two tables, Authors and Books, with a one-to-many relationship where each book references its author. Then insert sample data and write a query to list all books with their authors.

📋 What You'll Learn

Create a table called Authors with columns AuthorID (primary key) and Name (text).

Create a table called Books with columns BookID (primary key), Title (text), and AuthorID (foreign key referencing Authors.AuthorID).

Insert exactly two authors: AuthorID 1 with Name 'Jane Austen' and AuthorID 2 with Name 'Mark Twain'.

Insert exactly three books: 'Pride and Prejudice' by Jane Austen, 'Emma' by Jane Austen, and 'Adventures of Huckleberry Finn' by Mark Twain.

Write a query to select the book Title and the author's Name by joining the two tables.

💡 Why This Matters

🌍 Real World

One-to-many relationships are common in databases, such as customers and orders, authors and books, or teachers and students.

💼 Career

Understanding how to design and query one-to-many relationships is essential for database developers, data analysts, and backend engineers.

1

Create the Authors table

Write a SQL statement to create a table called Authors with columns AuthorID as an integer primary key and Name as text.

SQL

-- Create the Authors table with AuthorID as primary key and Name as text
-- Your code here

Hint

Use CREATE TABLE with the column definitions including PRIMARY KEY for AuthorID.

2

Create the Books table with foreign key

Write a SQL statement to create a table called Books with columns BookID as an integer primary key, Title as text, and AuthorID as an integer foreign key referencing Authors.AuthorID.

SQL

CREATE TABLE Authors (
    AuthorID INTEGER PRIMARY KEY,
    Name TEXT
);
-- Create the Books table with BookID as primary key, Title as text, and AuthorID as foreign key
-- Your code here

Hint

Remember to add FOREIGN KEY constraint for AuthorID referencing Authors table.

3

Insert sample data into Authors and Books

Insert two authors into Authors: (1, 'Jane Austen') and (2, 'Mark Twain'). Then insert three books into Books: 'Pride and Prejudice' with AuthorID 1, 'Emma' with AuthorID 1, and 'Adventures of Huckleberry Finn' with AuthorID 2.

SQL

CREATE TABLE Authors (
    AuthorID INTEGER PRIMARY KEY,
    Name TEXT
);

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT,
    AuthorID INTEGER,
    FOREIGN KEY (AuthorID) REFERENCES Authors(AuthorID)
);
-- Insert authors and books data
-- Your code here

Hint

Use INSERT INTO with exact values for both tables.

4

Query books with their authors

Write a SQL query to select the Title of each book and the Name of its author by joining the Books and Authors tables on AuthorID.

SQL

CREATE TABLE Authors (
    AuthorID INTEGER PRIMARY KEY,
    Name TEXT
);

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT,
    AuthorID INTEGER,
    FOREIGN KEY (AuthorID) REFERENCES Authors(AuthorID)
);

INSERT INTO Authors (AuthorID, Name) VALUES (1, 'Jane Austen');
INSERT INTO Authors (AuthorID, Name) VALUES (2, 'Mark Twain');

INSERT INTO Books (BookID, Title, AuthorID) VALUES (1, 'Pride and Prejudice', 1);
INSERT INTO Books (BookID, Title, AuthorID) VALUES (2, 'Emma', 1);
INSERT INTO Books (BookID, Title, AuthorID) VALUES (3, 'Adventures of Huckleberry Finn', 2);
-- Write a SELECT query joining Books and Authors to get Title and Name
-- Your code here

Hint

Use JOIN to connect Books and Authors on AuthorID and select Title and Name.

Practice

1. What does a one-to-many relationship in a database mean?

easy

A. One record in a table relates to many records in another table

B. Many records in a table relate to one record in the same table

C. One record relates to exactly one record in another table

D. Many records relate to many records in another table

Solution

1. Step 1: Understand relationship types

   A one-to-many relationship means one record in a table connects to multiple records in another table.

2. Step 2: Match definition to options

   One record in a table relates to many records in another table correctly describes this as one record relating to many records in another table.

3. Final Answer:

   One record in a table relates to many records in another table -> Option A

4. Quick Check:

   One-to-many = one record to many records [OK]

Hint: One-to-many means one record links to many records [OK]

Common Mistakes:

• Confusing one-to-many with many-to-many
• Thinking one-to-many means one record links to one record
• Mixing up the direction of the relationship

2. Which SQL statement correctly creates a foreign key for a one-to-many relationship from Orders to Customers?

easy

A. ALTER TABLE Customers ADD FOREIGN KEY (OrderID) REFERENCES Orders(OrderID);

B. ALTER TABLE Customers ADD FOREIGN KEY (CustomerID) REFERENCES Orders(OrderID);

C. ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID);

D. ALTER TABLE Orders ADD FOREIGN KEY (OrderID) REFERENCES Customers(CustomerID);

Solution

1. Step 1: Identify the 'many' and 'one' tables

   Orders is the 'many' side, Customers is the 'one' side in a one-to-many relationship.

2. Step 2: Add foreign key in 'many' table referencing 'one' table

   The foreign key should be in Orders referencing Customers, so ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID); is correct.

3. Final Answer:

   ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID); -> Option C

4. Quick Check:

   Foreign key in 'many' table references 'one' table [OK]

Hint: Foreign key goes in 'many' table pointing to 'one' table [OK]

Common Mistakes:

• Placing foreign key in the 'one' table instead of 'many'
• Referencing wrong columns between tables
• Mixing table names in foreign key definition

3. Given these tables:
Customers(CustomerID, Name)
Orders(OrderID, CustomerID, Amount)
What will this query return?

SELECT Customers.Name, COUNT(Orders.OrderID) AS OrderCount FROM Customers LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID GROUP BY Customers.Name;

medium

A. List of customers who have placed at least one order

B. List of customers with total amount spent on orders

C. List of orders with customer names repeated for each order

D. List of customers with the number of orders each placed, including customers with zero orders

Solution

1. Step 1: Understand the LEFT JOIN usage

   LEFT JOIN keeps all customers, even those without matching orders.

2. Step 2: COUNT(Orders.OrderID) counts orders per customer

   Grouping by customer name counts how many orders each customer has, zero if none.

3. Final Answer:

   List of customers with the number of orders each placed, including customers with zero orders -> Option D

4. Quick Check:

   LEFT JOIN + COUNT = all customers with order counts [OK]

Hint: LEFT JOIN + COUNT counts all, including zero matches [OK]

Common Mistakes:

• Thinking COUNT counts total amount spent
• Assuming only customers with orders appear
• Confusing JOIN types and their effects

4. You wrote this SQL to create a one-to-many relationship:

CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT, FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID));

But you get an error. What is the most likely cause?

medium

A. OrderID should not be primary key in Orders

B. Customers table does not exist or CustomerID is not a primary key

C. Foreign key should be in Customers table, not Orders

D. CustomerID column type must be VARCHAR, not INT

Solution

1. Step 1: Check foreign key reference validity

   Foreign key must reference an existing table and a primary or unique key column.

2. Step 2: Verify Customers table and CustomerID key

   If Customers table or CustomerID primary key is missing, error occurs.

3. Final Answer:

   Customers table does not exist or CustomerID is not a primary key -> Option B

4. Quick Check:

   Foreign key references must exist and be keys [OK]

Hint: Foreign key target must exist and be primary/unique key [OK]

Common Mistakes:

• Assuming foreign key can reference non-key columns
• Placing foreign key in wrong table
• Mismatching data types between foreign key and referenced key

5. You have two tables:
Authors(AuthorID, Name)
Books(BookID, Title, AuthorID)
You want to find authors who have written more than 3 books. Which query is correct?

hard

A. SELECT Name FROM Authors JOIN Books ON Authors.AuthorID = Books.AuthorID GROUP BY Name HAVING COUNT(BookID) > 3;

B. SELECT Name FROM Authors LEFT JOIN Books ON Authors.AuthorID = Books.AuthorID WHERE COUNT(BookID) > 3;

C. SELECT Name FROM Books GROUP BY AuthorID HAVING COUNT(BookID) > 3;

D. SELECT Name FROM Authors WHERE AuthorID IN (SELECT AuthorID FROM Books WHERE COUNT(BookID) > 3);

Solution

1. Step 1: Join Authors and Books on AuthorID

   We join to connect authors with their books.

2. Step 2: Group by author name and filter by book count

   Use GROUP BY Name and HAVING COUNT(BookID) > 3 to find authors with more than 3 books.

3. Final Answer:

   SELECT Name FROM Authors JOIN Books ON Authors.AuthorID = Books.AuthorID GROUP BY Name HAVING COUNT(BookID) > 3; -> Option A

4. Quick Check:

   JOIN + GROUP BY + HAVING filters authors by book count [OK]

Hint: Use GROUP BY and HAVING to filter by count in one-to-many [OK]

Common Mistakes:

• Using WHERE with aggregate functions instead of HAVING
• Missing GROUP BY clause
• Using LEFT JOIN but filtering with WHERE on aggregate