developer-roadmap/src/data/courses/sql-mastery/chapters/subqueries-and-ctes/lessons/above-average-books.md

title	description	order	type	setup
Above Average Books	Practice using scalar subqueries to find books above average price in their category	140	challenge	```sql CREATE TABLE book ( id INT PRIMARY KEY, title VARCHAR(100), category VARCHAR(50), price DECIMAL(10,2), published_date DATE ); INSERT INTO book (id, title, category, price, published_date) VALUES (1, 'SQL Basics', 'Programming', 29.99, '2024-01-15'), (2, 'Advanced SQL', 'Programming', 49.99, '2024-02-20'), (3, 'Data Science', 'Data Analysis', 39.99, '2024-01-10'), (4, 'Web Development', 'Programming', 34.99, '2024-03-01'), (5, 'Statistics 101', 'Data Analysis', 24.99, '2024-02-15'), (6, 'Machine Learning', 'Data Analysis', 44.99, '2024-03-15'), (7, 'Database Design', 'Programming', 39.99, '2024-01-20'), (8, 'Python Basics', 'Programming', 29.99, '2024-02-01'); ```

The bookstore manager wants to identify books that are priced above the average price in their respective categories, but only for books published in 2024. This information will help in adjusting pricing strategies.

Given the following data in table book:

id	title	category	price	published_date
1	SQL Basics	Programming	29.99	2024-01-15
2	Advanced SQL	Programming	49.99	2024-02-20
3	Data Science	Data Analysis	39.99	2024-01-10
4	Web Development	Programming	34.99	2024-03-01
5	Statistics 101	Data Analysis	24.99	2024-02-15
6	Machine Learning	Data Analysis	44.99	2024-03-15
7	Database Design	Programming	39.99	2024-01-20
8	Python Basics	Programming	29.99	2024-02-01

Write a query that shows:

• Book title
• Category
• Book price
• Category average price (rounded to 2 decimal places)
• Price difference from category average (rounded to 2 decimal places)

Only include books that are:

1. Published in 2024
2. Priced above their category's average price

Expected Output

title	category	price	category_avg	price_diff
Machine Learning	Data Analysis	44.99	36.66	8.33
Data Science	Data Analysis	39.99	36.66	3.33
Advanced SQL	Programming	49.99	36.99	13.00
Database Design	Programming	39.99	36.99	3.00

Solution

SELECT
    b1.title,
    b1.category,
    b1.price,
    ROUND(avg_price, 2) AS category_avg,
    ROUND(b1.price - avg_price, 2) AS price_diff
FROM book b1
INNER JOIN (
    -- Subquery to calculate average price per category for 2024
    SELECT
        category,
        AVG(price) AS avg_price
    FROM book
    WHERE EXTRACT(YEAR FROM published_date) = 2024
    GROUP BY category
) AS ca ON b1.category = ca.category
WHERE
    EXTRACT(YEAR FROM b1.published_date) = 2024 -- Filter for books published in 2024
    AND b1.price > ca.avg_price                 -- Filter books above category average
ORDER BY
    b1.category, price_diff DESC;               -- Sort results