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medium Library Book Checkouts First Year

Problem Statement

Table: Checkouts

This table records the checkouts of books in a library. Each record includes a checkout ID, book ID, the year the book was checked out, the number of times it was checked out that year, and the price per checkout.

+----------------+-------+
| Column Name    | Type  |
+----------------+-------+
| checkout_id    | int   |
| book_id        | int   |
| year           | int   |
| checkouts      | int   |
| price          | int   |
+----------------+-------+
(checkout_id, year) is the primary key of this table.
book_id is a foreign key to the Books table.
Each row of this table shows a checkout record of the book book_id in a certain year.
Note that the price is per checkout.

Table: Books

This table lists all the books available in the library.

+------------+-----------+
| Column Name | Type     |
+------------+-----------+
| book_id     | int      |
| book_title  | varchar  |
+------------+-----------+
book_id is the primary key of this table.
Each row of this table indicates the title of each book.

Develop a solution to select the book id, the first year the book was checked out, the total number of checkouts, and the price for the first year of every book checked out.

Return the resulting table in any order.

Example

Input:

Checkouts table:
+-------------+---------+-------+----------+-------+
| checkout_id | book_id | year  | checkouts| price |
+-------------+---------+-------+----------+-------+ 
| 1           | 100     | 2008  | 40       | 2     |
| 2           | 100     | 2009  | 50       | 2     |
| 3           | 200     | 2011  | 60       | 3     |
+-------------+---------+-------+----------+-------+

Books table:
+---------+-----------------+
| book_id | book_title      |
+---------+-----------------+
| 100     | War and Peace   |
| 200     | The Great Gatsby|
| 300     | Moby Dick       |
+---------+-----------------+

Output:

+---------+------------+----------+-------+
| book_id | first_year | checkouts| price |
+---------+------------+----------+-------+ 
| 100     | 2008       | 40       | 2     |
| 200     | 2011       | 60       | 3     |
+---------+------------+----------+-------+

Try It Yourself

java

-- TODO: Write your user queries here

Solution

To identify each book's first year of checkout, along with the total number of checkouts and the price per checkout for that year, we can follow a systematic approach using SQL's aggregation and join capabilities. This ensures accurate retrieval of the desired information by leveraging Common Table Expressions (CTEs) and appropriate filtering.

Determine the First Year of Checkout for Each Book: Identify the earliest year each book was checked out by finding the minimum year associated with each book_id.
Retrieve Checkout Details for the First Year: Obtain the number of checkouts and the price per checkout for each book in its first year of checkout.
Order the Results: Sort the final output by book_id to present the data in an organized manner.

SQL Query

WITH FirstYear AS (
    SELECT book_id, MIN(year) AS first_year
    FROM Checkouts
    GROUP BY book_id
)
SELECT f.book_id, f.first_year, c.checkouts, c.price
FROM FirstYear f
JOIN Checkouts c ON f.book_id = c.book_id AND f.first_year = c.year
ORDER BY book_id;

Step-by-Step Approach

Step 1: Determine the First Year of Checkout for Each Book

Identify the earliest year each book was checked out by finding the minimum year associated with each book_id.

SQL Query:

WITH FirstYear AS (
    SELECT book_id, MIN(year) AS first_year
    FROM Checkouts
    GROUP BY book_id
)

Explanation:

WITH FirstYear AS (...):
- Creates a Common Table Expression (CTE) named FirstYear that stores the earliest checkout year for each book.
SELECT book_id, MIN(year) AS first_year:
- Selects each book_id and calculates the minimum year it was checked out.
FROM Checkouts:
- Uses the Checkouts table as the data source.
GROUP BY book_id:
- Groups the records by book_id to perform the aggregation, ensuring that the minimum year is calculated for each individual book.

Output After Step 1:

+---------+------------+
| book_id | first_year |
+---------+------------+
| 100     | 2008       |
| 200     | 2011       |
+---------+------------+

Step 2: Retrieve Checkout Details for the First Year

For each book, obtain the number of checkouts and the price per checkout corresponding to its first year of checkout.

SQL Query:

SELECT f.book_id, f.first_year, c.checkouts, c.price
FROM FirstYear f
JOIN Checkouts c ON f.book_id = c.book_id AND f.first_year = c.year

Explanation:

SELECT f.book_id, f.first_year, c.checkouts, c.price:
- Selects the book_id, the determined first_year, the number of checkouts, and the price per checkout.
FROM FirstYear f:
- Uses the FirstYear CTE as the primary table to access each book's first checkout year.
JOIN Checkouts c ON f.book_id = c.book_id AND f.first_year = c.year:
- Joins the Checkouts table to fetch the corresponding checkout details for the first year of each book.
- f.book_id = c.book_id AND f.first_year = c.year:
  - Ensures that only the records matching the first year are retrieved, effectively linking each book to its first checkout record.

Output After Step 2:

+---------+------------+----------+-------+
| book_id | first_year | checkouts| price |
+---------+------------+----------+-------+
| 100     | 2008       | 40       | 2     |
| 200     | 2011       | 60       | 3     |
+---------+------------+----------+-------+

Step 3: Order the Results by Book ID

Sort the final output in ascending order based on book_id to ensure organized presentation.

SQL Query:

ORDER BY book_id;

Explanation:

ORDER BY book_id:
- Sorts the results by book_id in ascending order, making the output orderly and easy to interpret.

Final Output:

+---------+------------+----------+-------+
| book_id | first_year | checkouts| price |
+---------+------------+----------+-------+
| 100     | 2008       | 40       | 2     |
| 200     | 2011       | 60       | 3     |
+---------+------------+----------+-------+

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