Multiple table JOINs in MySQL - Mini Project: Build & Apply

Multiple Table JOINs in MySQL

📖 Scenario: You are managing a small online bookstore database. The database has three tables: authors, books, and sales. You want to find out which authors have sold books, and details about those sales.

🎯 Goal: Build a MySQL query that joins the authors, books, and sales tables to list each author's name, the title of their book, and the number of copies sold.

📋 What You'll Learn

Create a table called authors with columns author_id (INT) and author_name (VARCHAR).

Create a table called books with columns book_id (INT), title (VARCHAR), and author_id (INT).

Create a table called sales with columns sale_id (INT), book_id (INT), and copies_sold (INT).

Write a SELECT query that uses JOINs to combine these tables and show author_name, title, and copies_sold.

Use INNER JOINs to only show books that have sales.

💡 Why This Matters

🌍 Real World

Online bookstores and many other businesses use multiple related tables to organize data about products, customers, and sales.

💼 Career

Understanding how to join multiple tables is essential for database querying roles, data analysis, and backend development.

1

Create the authors table and insert data

Create a table called authors with columns author_id (INT) and author_name (VARCHAR(50)). Insert these exact rows: (1, 'Jane Austen'), (2, 'Mark Twain'), (3, 'J.K. Rowling').

MySQL

-- Create the authors table and insert data
-- Your code here

Hint

Use CREATE TABLE authors (author_id INT, author_name VARCHAR(50)); and then INSERT INTO authors with the given values.

2

Create the books table and insert data

Create a table called books with columns book_id (INT), title (VARCHAR(100)), and author_id (INT). Insert these exact rows: (101, 'Pride and Prejudice', 1), (102, 'Adventures of Huckleberry Finn', 2), (103, 'Harry Potter and the Sorcerer''s Stone', 3).

MySQL

CREATE TABLE authors (
  author_id INT,
  author_name VARCHAR(50)
);

INSERT INTO authors (author_id, author_name) VALUES
(1, 'Jane Austen'),
(2, 'Mark Twain'),
(3, 'J.K. Rowling');

-- Create the books table and insert data
-- Your code here

Hint

Use CREATE TABLE books (book_id INT, title VARCHAR(100), author_id INT); and then INSERT INTO books with the given values.

3

Create the sales table and insert data

Create a table called sales with columns sale_id (INT), book_id (INT), and copies_sold (INT). Insert these exact rows: (1001, 101, 500), (1002, 102, 300), (1003, 103, 1000).

MySQL

CREATE TABLE authors (
  author_id INT,
  author_name VARCHAR(50)
);

INSERT INTO authors (author_id, author_name) VALUES
(1, 'Jane Austen'),
(2, 'Mark Twain'),
(3, 'J.K. Rowling');

CREATE TABLE books (
  book_id INT,
  title VARCHAR(100),
  author_id INT
);

INSERT INTO books (book_id, title, author_id) VALUES
(101, 'Pride and Prejudice', 1),
(102, 'Adventures of Huckleberry Finn', 2),
(103, 'Harry Potter and the Sorcerer''s Stone', 3);

-- Create the sales table and insert data
-- Your code here

Hint

Use CREATE TABLE sales (sale_id INT, book_id INT, copies_sold INT); and then INSERT INTO sales with the given values.

4

Write a JOIN query to list authors, book titles, and copies sold

Write a SELECT query that uses INNER JOINs to combine authors, books, and sales. Select author_name, title, and copies_sold. Use the exact JOIN syntax: FROM authors JOIN books ON authors.author_id = books.author_id JOIN sales ON books.book_id = sales.book_id.

MySQL

CREATE TABLE authors (
  author_id INT,
  author_name VARCHAR(50)
);

INSERT INTO authors (author_id, author_name) VALUES
(1, 'Jane Austen'),
(2, 'Mark Twain'),
(3, 'J.K. Rowling');

CREATE TABLE books (
  book_id INT,
  title VARCHAR(100),
  author_id INT
);

INSERT INTO books (book_id, title, author_id) VALUES
(101, 'Pride and Prejudice', 1),
(102, 'Adventures of Huckleberry Finn', 2),
(103, 'Harry Potter and the Sorcerer''s Stone', 3);

CREATE TABLE sales (
  sale_id INT,
  book_id INT,
  copies_sold INT
);

INSERT INTO sales (sale_id, book_id, copies_sold) VALUES
(1001, 101, 500),
(1002, 102, 300),
(1003, 103, 1000);

-- Write the JOIN query below
-- Your code here

Hint

Use INNER JOINs to connect the tables by their IDs and select the requested columns.

Practice

1. What is the main purpose of using JOIN with multiple tables in MySQL?

easy

A. To delete rows from multiple tables at once

B. To combine rows from two or more tables based on a related column

C. To create a new table from existing tables

D. To update multiple tables simultaneously

Solution

1. Step 1: Understand the role of JOIN

   JOIN is used to combine rows from two or more tables based on a related column, allowing us to see related data together.

2. Step 2: Compare with other options

   Deleting, creating, or updating tables are different SQL operations and not the main purpose of JOIN.

3. Final Answer:

   To combine rows from two or more tables based on a related column -> Option B

4. Quick Check:

   JOIN combines tables = To combine rows [OK]

Hint: JOIN connects tables by matching columns [OK]

Common Mistakes:

• Confusing JOIN with DELETE or UPDATE operations
• Thinking JOIN creates new tables permanently
• Assuming JOIN works without specifying related columns

2. Which of the following is the correct syntax to join two tables employees and departments on the column department_id?

easy

A. SELECT * FROM employees JOIN departments ON employees.department_id = departments.department_id;

B. SELECT * FROM employees JOIN departments WHERE employees.department_id = departments.department_id;

C. SELECT * FROM employees, departments ON employees.department_id = departments.department_id;

D. SELECT * FROM employees JOIN departments USING employees.department_id;

Solution

1. Step 1: Check JOIN syntax with ON clause

   The correct syntax uses JOIN followed by ON to specify the matching columns: JOIN departments ON employees.department_id = departments.department_id.

2. Step 2: Identify errors in other options

   SELECT * FROM employees JOIN departments WHERE employees.department_id = departments.department_id; uses WHERE instead of ON, which is incorrect for JOIN conditions. SELECT * FROM employees, departments ON employees.department_id = departments.department_id; uses ON without JOIN keyword properly. SELECT * FROM employees JOIN departments USING employees.department_id; uses USING incorrectly with full column name.

3. Final Answer:

   SELECT * FROM employees JOIN departments ON employees.department_id = departments.department_id; -> Option A

4. Quick Check:

   JOIN with ON = SELECT * FROM employees JOIN departments ON employees.department_id = departments.department_id; [OK]

Hint: JOIN needs ON, not WHERE, to link tables [OK]

Common Mistakes:

• Using WHERE instead of ON for JOIN condition
• Using USING with full table.column syntax
• Omitting JOIN keyword or ON clause

3. Given the tables:
students(id, name, class_id)
classes(id, class_name)
What is the output of this query?

SELECT students.name, classes.class_name FROM students JOIN classes ON students.class_id = classes.id WHERE classes.class_name = 'Math';

medium

A. Empty result because of missing WHERE clause

B. List of all students with their class names

C. List of class names only

D. List of student names who are in the Math class

Solution

1. Step 1: Understand the JOIN and WHERE clause

   The JOIN connects students with their classes by matching class_id and id. The WHERE clause filters only classes named 'Math'.

2. Step 2: Determine the output columns and rows

   The SELECT returns student names and class names only for students in 'Math' class, so the output is a list of those students with 'Math' as class_name.

3. Final Answer:

   List of student names who are in the Math class -> Option D

4. Quick Check:

   JOIN + WHERE filters = students in Math [OK]

Hint: WHERE filters after JOIN to limit rows [OK]

Common Mistakes:

• Thinking it returns all students regardless of class
• Confusing output columns with only class names
• Assuming WHERE is missing and causes empty result

4. Identify the error in this query joining orders and customers:

SELECT orders.id, customers.name FROM orders JOIN customers WHERE orders.customer_id = customers.id;

medium

A. Missing ON keyword before join condition

B. Using WHERE instead of HAVING

C. Incorrect table names

D. Missing SELECT keyword

Solution

1. Step 1: Check JOIN syntax

   The JOIN clause must be followed by ON to specify the join condition. Here, WHERE is incorrectly used instead of ON.

2. Step 2: Confirm other parts are correct

   Table names and SELECT keyword are correct. HAVING is not used for join conditions.

3. Final Answer:

   Missing ON keyword before join condition -> Option A

4. Quick Check:

   JOIN needs ON, not WHERE [OK]

Hint: JOIN requires ON before condition, not WHERE [OK]

Common Mistakes:

• Replacing ON with WHERE in JOIN
• Confusing WHERE with HAVING clause
• Ignoring syntax errors in JOIN statements

5. You have three tables:
products(id, name, category_id)
categories(id, category_name)
sales(product_id, quantity)
Which query correctly lists each product's name, its category name, and total quantity sold?

hard

A. SELECT p.name, c.category_name, SUM(s.quantity) FROM products p LEFT JOIN categories c ON p.category_id = c.id LEFT JOIN sales s ON p.id = s.product_id GROUP BY p.name;

B. SELECT p.name, c.category_name, s.quantity FROM products p JOIN categories c ON p.category_id = c.id JOIN sales s ON p.id = s.product_id;

C. SELECT p.name, c.category_name, SUM(s.quantity) FROM products p JOIN categories c ON p.category_id = c.id JOIN sales s ON p.id = s.product_id GROUP BY p.name, c.category_name;

D. SELECT p.name, c.category_name, SUM(s.quantity) FROM products p, categories c, sales s WHERE p.category_id = c.id AND p.id = s.product_id;

Solution

1. Step 1: Understand the requirement

   We need product name, category name, and total quantity sold (sum of sales.quantity) grouped by product and category.

2. Step 2: Analyze each option

   SELECT p.name, c.category_name, SUM(s.quantity) FROM products p JOIN categories c ON p.category_id = c.id JOIN sales s ON p.id = s.product_id GROUP BY p.name, c.category_name; uses JOINs with ON clauses correctly and groups by product and category names, summing quantity. SELECT p.name, c.category_name, s.quantity FROM products p JOIN categories c ON p.category_id = c.id JOIN sales s ON p.id = s.product_id; misses aggregation. SELECT p.name, c.category_name, SUM(s.quantity) FROM products p, categories c, sales s WHERE p.category_id = c.id AND p.id = s.product_id; uses implicit joins but misses GROUP BY. SELECT p.name, c.category_name, SUM(s.quantity) FROM products p LEFT JOIN categories c ON p.category_id = c.id LEFT JOIN sales s ON p.id = s.product_id GROUP BY p.name; uses LEFT JOIN but groups only by product name, missing category grouping.

3. Final Answer:

   SELECT p.name, c.category_name, SUM(s.quantity) FROM products p JOIN categories c ON p.category_id = c.id JOIN sales s ON p.id = s.product_id GROUP BY p.name, c.category_name; -> Option C

4. Quick Check:

   JOIN + GROUP BY + SUM = SELECT p.name, c.category_name, SUM(s.quantity) FROM products p JOIN categories c ON p.category_id = c.id JOIN sales s ON p.id = s.product_id GROUP BY p.name, c.category_name; [OK]

Hint: Use JOIN with GROUP BY and SUM for totals [OK]

Common Mistakes:

• Missing GROUP BY when using SUM
• Using WHERE instead of JOIN ON
• Grouping by only one column when multiple needed