GROUP BY with aggregate functions in SQL - Mini Project: Build & Apply

GROUP BY with aggregate functions

📖 Scenario: You are managing a small bookstore database. You want to find out how many books are available in each genre and the average price of books in each genre.

🎯 Goal: Build a SQL query that groups books by their genre and calculates the total number of books and the average price for each genre.

📋 What You'll Learn

Create a table called books with columns id, title, genre, and price.

Insert the exact data rows provided into the books table.

Write a SQL query that groups the books by genre.

Use aggregate functions COUNT() and AVG() to find the number of books and average price per genre.

💡 Why This Matters

🌍 Real World

Grouping and summarizing data is common in business reports, like sales by category or average ratings by product type.

💼 Career

Database analysts and developers often write GROUP BY queries with aggregate functions to generate insights from data.

1

Create the books table and insert data

Create a table called books with columns id (integer), title (text), genre (text), and price (decimal). Then insert these exact rows into books: (1, 'The Hobbit', 'Fantasy', 10.99), (2, '1984', 'Dystopian', 8.99), (3, 'The Catcher in the Rye', 'Classic', 6.99), (4, 'The Lord of the Rings', 'Fantasy', 15.99), (5, 'Brave New World', 'Dystopian', 9.99).

SQL

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

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Set up the grouping column

Write a SQL query that selects the genre column from the books table and groups the results by genre.

SQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  genre TEXT,
  price DECIMAL
);

INSERT INTO books (id, title, genre, price) VALUES
(1, 'The Hobbit', 'Fantasy', 10.99),
(2, '1984', 'Dystopian', 8.99),
(3, 'The Catcher in the Rye', 'Classic', 6.99),
(4, 'The Lord of the Rings', 'Fantasy', 15.99),
(5, 'Brave New World', 'Dystopian', 9.99);

-- Add GROUP BY clause to group by genre
SELECT genre
FROM books
GROUP BY genre
-- Your code here

Hint

Use GROUP BY genre to group rows by genre.

3

Add aggregate functions to count books and average price

Modify the SQL query to select genre, the count of books as book_count using COUNT(*), and the average price as avg_price using AVG(price). Keep grouping by genre.

SQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  genre TEXT,
  price DECIMAL
);

INSERT INTO books (id, title, genre, price) VALUES
(1, 'The Hobbit', 'Fantasy', 10.99),
(2, '1984', 'Dystopian', 8.99),
(3, 'The Catcher in the Rye', 'Classic', 6.99),
(4, 'The Lord of the Rings', 'Fantasy', 15.99),
(5, 'Brave New World', 'Dystopian', 9.99);

-- Add COUNT(*) as book_count and AVG(price) as avg_price
SELECT genre, COUNT(*) AS book_count, AVG(price) AS avg_price
FROM books
GROUP BY genre
-- Your code here

Hint

Use COUNT(*) AS book_count and AVG(price) AS avg_price in the SELECT clause.

4

Complete the query with ordering

Add an ORDER BY clause to the query to sort the results by book_count in descending order.

SQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  genre TEXT,
  price DECIMAL
);

INSERT INTO books (id, title, genre, price) VALUES
(1, 'The Hobbit', 'Fantasy', 10.99),
(2, '1984', 'Dystopian', 8.99),
(3, 'The Catcher in the Rye', 'Classic', 6.99),
(4, 'The Lord of the Rings', 'Fantasy', 15.99),
(5, 'Brave New World', 'Dystopian', 9.99);

-- Add ORDER BY book_count DESC to sort results
SELECT genre, COUNT(*) AS book_count, AVG(price) AS avg_price
FROM books
GROUP BY genre
ORDER BY book_count DESC
-- Your code here

Hint

Use ORDER BY book_count DESC to sort genres by number of books from highest to lowest.

Practice

1. What does the GROUP BY clause do in an SQL query?

easy

A. It deletes duplicate rows from the table.

B. It sorts the rows in ascending order.

C. It groups rows that have the same values in specified columns.

D. It filters rows based on a condition.

Solution

1. Step 1: Understand the purpose of GROUP BY

   The GROUP BY clause is used to group rows that share the same values in one or more columns.

2. Step 2: Differentiate from other clauses

   Sorting is done by ORDER BY, filtering by WHERE, and removing duplicates by DISTINCT, not GROUP BY.

3. Final Answer:

   It groups rows that have the same values in specified columns. -> Option C

4. Quick Check:

   GROUP BY = groups rows by column values [OK]

Hint: GROUP BY groups rows by column values, not sorting or filtering [OK]

Common Mistakes:

• Confusing GROUP BY with ORDER BY
• Thinking GROUP BY filters rows
• Assuming GROUP BY removes duplicates

2. Which of the following SQL queries correctly uses GROUP BY to count employees per department?

easy

A. SELECT department, COUNT(*) FROM employees GROUP BY.;

B. SELECT department, COUNT(*) FROM employees GROUP BY department.;

C. SELECT department, COUNT(*) FROM employees WHERE department GROUP BY.;

D. SELECT department, COUNT(*) FROM employees.;

Solution

1. Step 1: Check the syntax of GROUP BY usage

   The correct syntax requires specifying the column after GROUP BY and using aggregate functions properly.

2. Step 2: Analyze each option

   Only SELECT department, COUNT(*) FROM employees GROUP BY department; correctly groups by department and counts employees. The other options have syntax errors: missing column after GROUP BY, no GROUP BY clause, or invalid WHERE syntax.

3. Final Answer:

   SELECT department, COUNT(*) FROM employees GROUP BY department; -> Option B

4. Quick Check:

   GROUP BY column + aggregate function = correct syntax [OK]

Hint: GROUP BY must be followed by column names, aggregate functions used outside [OK]

Common Mistakes:

• Omitting column after GROUP BY
• Using WHERE incorrectly with GROUP BY
• Missing aggregate function with GROUP BY

3. Given the table sales with columns region and amount, what is the result of this query?

SELECT region, SUM(amount) FROM sales GROUP BY region;

medium

A. A list of all sales amounts without grouping.

B. A list of regions with the average sales amount.

C. An error because SUM() cannot be used with GROUP BY.

D. A list of regions with the total sales amount for each region.

Solution

1. Step 1: Understand the query components

   The query groups rows by region and calculates the sum of amount for each group.

2. Step 2: Determine the output

   The output will show each region once with the total sales amount summed up.

3. Final Answer:

   A list of regions with the total sales amount for each region. -> Option D

4. Quick Check:

   GROUP BY region + SUM(amount) = total per region [OK]

Hint: SUM with GROUP BY gives total per group, not average or error [OK]

Common Mistakes:

• Confusing SUM with AVG
• Expecting no grouping effect
• Thinking SUM causes error with GROUP BY

4. Identify the error in this SQL query:

SELECT department, AVG(salary) FROM employees WHERE department GROUP BY department;

medium

A. Missing condition after WHERE clause.

B. AVG() cannot be used with GROUP BY.

C. GROUP BY should come before WHERE.

D. department cannot be selected with AVG().

Solution

1. Step 1: Analyze the WHERE clause

   The WHERE clause requires a condition, but here it only has 'department' which is incomplete and invalid.

2. Step 2: Check GROUP BY and AVG usage

   GROUP BY after WHERE is correct, and AVG() can be used with GROUP BY, so no error there.

3. Final Answer:

   Missing condition after WHERE clause. -> Option A

4. Quick Check:

   WHERE needs a condition, not just a column name [OK]

Hint: WHERE must have a condition; column alone is invalid [OK]

Common Mistakes:

• Using WHERE without condition
• Thinking GROUP BY order is wrong
• Believing AVG() can't be grouped

5. You have a products table with columns category, price, and stock. Which query shows the average price and total stock for each category, but only for categories with more than 10 products?

hard

A. SELECT category, AVG(price), SUM(stock) FROM products GROUP BY category HAVING COUNT(*) > 10;

B. SELECT category, AVG(price), SUM(stock) FROM products WHERE COUNT(*) > 10 GROUP BY category;

C. SELECT category, AVG(price), SUM(stock) FROM products GROUP BY category WHERE COUNT(*) > 10;

D. SELECT category, AVG(price), SUM(stock) FROM products HAVING COUNT(*) > 10 GROUP BY category;

Solution

1. Step 1: Understand filtering groups with HAVING

   To filter groups based on aggregate conditions, use HAVING after GROUP BY.

2. Step 2: Analyze each option's clause order

   Only SELECT category, AVG(price), SUM(stock) FROM products GROUP BY category HAVING COUNT(*) > 10; correctly uses GROUP BY then HAVING. Using WHERE with COUNT(*) is invalid (WHERE processes rows before grouping), and placing HAVING before GROUP BY or incorrect clause ordering is invalid syntax.

3. Final Answer:

   SELECT category, AVG(price), SUM(stock) FROM products GROUP BY category HAVING COUNT(*) > 10; -> Option A

4. Quick Check:

   Use HAVING to filter groups after GROUP BY [OK]

Hint: Use HAVING after GROUP BY to filter groups by aggregate [OK]

Common Mistakes:

• Using WHERE to filter aggregate results
• Placing HAVING before GROUP BY
• Confusing clause order in SQL