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  • src
    • main/kotlin/g3601_3700/s3642_find_books_with_polarized_opinions
    • test/kotlin/g3601_3700/s3642_find_books_with_polarized_opinions

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‎src/main/kotlin/g3601_3700/s3642_find_books_with_polarized_opinions/readme.md

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3642\. Find Books with Polarized Opinions
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Easy
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Table: `books`
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+-------------+---------+
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| Column Name | Type |
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+-------------+---------+
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| book_id | int |
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| title | varchar |
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| author | varchar |
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| genre | varchar |
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| pages | int |
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+-------------+---------+
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book_id is the unique ID for this table. Each row contains information about a book including its genre and page count.
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Table: `reading_sessions`
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+----------------+---------+
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| Column Name | Type |
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+----------------+---------+
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| session_id | int |
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| book_id | int |
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| reader_name | varchar |
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| pages_read | int |
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| session_rating | int |
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+----------------+---------+
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session_id is the unique ID for this table. Each row represents a reading session where someone read a portion of a book. session_rating is on a scale of 1-5.
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Write a solution to find books that have **polarized opinions** - books that receive both very high ratings and very low ratings from different readers.
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* A book has polarized opinions if it has `at least one rating ≥ 4` and `at least one rating ≤ 2`
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* Only consider books that have **at least** `5` **reading sessions**
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* Calculate the **rating spread** as (`highest_rating - lowest_rating`)
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* Calculate the **polarization score** as the number of extreme ratings (`ratings ≤ 2 or ≥ 4`) divided by total sessions
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* **Only include** books where `polarization score ≥ 0.6` (at least `60%` extreme ratings)
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Return _the result table ordered by polarization score in **descending** order, then by title in **descending** order_.
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The result format is in the following example.
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**Example:**
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**Input:**
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books table:
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+---------+------------------------+---------------+----------+-------+
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| book_id | title | author | genre | pages |
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+---------+------------------------+---------------+----------+-------+
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| 1 | The Great Gatsby | F. Scott | Fiction | 180 |
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| 2 | To Kill a Mockingbird | Harper Lee | Fiction | 281 |
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| 3 | 1984 | George Orwell | Dystopian| 328 |
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| 4 | Pride and Prejudice | Jane Austen | Romance | 432 |
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| 5 | The Catcher in the Rye | J.D. Salinger | Fiction | 277 |
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+---------+------------------------+---------------+----------+-------+
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reading\_sessions table:
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+------------+---------+-------------+------------+----------------+
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| session_id | book_id | reader_name | pages_read | session_rating |
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+------------+---------+-------------+------------+----------------+
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| 1 | 1 | Alice | 50 | 5 |
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| 2 | 1 | Bob | 60 | 1 |
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| 3 | 1 | Carol | 40 | 4 |
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| 4 | 1 | David | 30 | 2 |
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| 5 | 1 | Emma | 45 | 5 |
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| 6 | 2 | Frank | 80 | 4 |
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| 7 | 2 | Grace | 70 | 4 |
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| 8 | 2 | Henry | 90 | 5 |
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| 9 | 2 | Ivy | 60 | 4 |
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| 10 | 2 | Jack | 75 | 4 |
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| 11 | 3 | Kate | 100 | 2 |
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| 12 | 3 | Liam | 120 | 1 |
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| 13 | 3 | Mia | 80 | 2 |
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| 14 | 3 | Noah | 90 | 1 |
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| 15 | 3 | Olivia | 110 | 4 |
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| 16 | 3 | Paul | 95 | 5 |
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| 17 | 4 | Quinn | 150 | 3 |
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| 18 | 4 | Ruby | 140 | 3 |
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| 19 | 5 | Sam | 80 | 1 |
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| 20 | 5 | Tara | 70 | 2 |
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+------------+---------+-------------+------------+----------------+
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**Output:**
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+---------+------------------+---------------+-----------+-------+---------------+--------------------+
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| book_id | title | author | genre | pages | rating_spread | polarization_score |
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+---------+------------------+---------------+-----------+-------+---------------+--------------------+
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| 1 | The Great Gatsby | F. Scott | Fiction | 180 | 4 | 1.00 |
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| 3 | 1984 | George Orwell | Dystopian | 328 | 4 | 1.00 |
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+---------+------------------+---------------+-----------+-------+---------------+--------------------+
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**Explanation:**
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* **The Great Gatsby (book\_id = 1):**
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* Has 5 reading sessions (meets minimum requirement)
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* Ratings: 5, 1, 4, 2, 5
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* Has ratings ≥ 4: 5, 4, 5 (3 sessions)
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* Has ratings ≤ 2: 1, 2 (2 sessions)
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* Rating spread: 5 - 1 = 4
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* Extreme ratings (≤2 or ≥4): All 5 sessions (5, 1, 4, 2, 5)
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* Polarization score: 5/5 = 1.00 (≥ 0.6, qualifies)
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* **1984 (book\_id = 3):**
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* Has 6 reading sessions (meets minimum requirement)
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* Ratings: 2, 1, 2, 1, 4, 5
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* Has ratings ≥ 4: 4, 5 (2 sessions)
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* Has ratings ≤ 2: 2, 1, 2, 1 (4 sessions)
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* Rating spread: 5 - 1 = 4
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* Extreme ratings (≤2 or ≥4): All 6 sessions (2, 1, 2, 1, 4, 5)
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* Polarization score: 6/6 = 1.00 (≥ 0.6, qualifies)
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* **Books not included:**
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* To Kill a Mockingbird (book\_id = 2): All ratings are 4-5, no low ratings (≤2)
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* Pride and Prejudice (book\_id = 4): Only 2 sessions (< 5 minimum)
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* The Catcher in the Rye (book\_id = 5): Only 2 sessions (< 5 minimum)
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The result table is ordered by polarization score in descending order, then by book title in descending order.

‎src/main/kotlin/g3601_3700/s3642_find_books_with_polarized_opinions/script.sql

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# Write your MySQL query statement below
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# #Easy #Database #2025_08_10_Time_490_ms_(100.00%)_Space_0.0_MB_(100.00%)
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WITH book_stats AS (
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SELECT
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book_id,
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COUNT(*) AS total_sessions,
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SUM(CASE WHEN session_rating <> 3 THEN 1 ELSE 0 END) AS extreme_ratings,
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MAX(session_rating) AS max_rating,
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MIN(session_rating) AS min_rating,
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SUM(CASE WHEN session_rating > 3 THEN 1 ELSE 0 END) AS high_ratings,
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SUM(CASE WHEN session_rating <= 2 THEN 1 ELSE 0 END) AS low_ratings
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FROM reading_sessions
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GROUP BY book_id
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)
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SELECT
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bs.book_id,
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b.title,
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b.author,
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b.genre,
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b.pages,
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(bs.max_rating - bs.min_rating) AS rating_spread,
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ROUND(bs.extreme_ratings * 1.0 / bs.total_sessions, 2) AS polarization_score
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FROM book_stats bs
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JOIN books b USING (book_id)
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WHERE
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bs.total_sessions >= 5
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AND bs.high_ratings > 0
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AND bs.low_ratings > 0
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AND (bs.extreme_ratings * 1.0 / bs.total_sessions) >= 0.6
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ORDER BY polarization_score DESC, b.title DESC;

‎src/test/kotlin/g3601_3700/s3642_find_books_with_polarized_opinions/MysqlTest.kt

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package g3601_3700.s3642_find_books_with_polarized_opinions
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import org.hamcrest.CoreMatchers.equalTo
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import org.hamcrest.MatcherAssert.assertThat
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import org.junit.jupiter.api.Test
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import org.zapodot.junit.db.annotations.EmbeddedDatabase
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import org.zapodot.junit.db.annotations.EmbeddedDatabaseTest
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import org.zapodot.junit.db.common.CompatibilityMode
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import java.io.BufferedReader
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import java.io.FileNotFoundException
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import java.io.FileReader
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import java.sql.SQLException
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import java.util.stream.Collectors
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import javax.sql.DataSource
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@EmbeddedDatabaseTest(
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compatibilityMode = CompatibilityMode.MySQL,
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initialSqls = [
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(
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"CREATE TABLE books (" +
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" book_id INT PRIMARY KEY," +
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" title VARCHAR(255)," +
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" author VARCHAR(255)," +
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" genre VARCHAR(50)," +
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" pages INT" +
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");" +
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"INSERT INTO books (book_id, title, author, genre, pages) VALUES" +
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"(1, 'The Great Gatsby', 'F. Scott', 'Fiction', 180)," +
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"(2, 'To Kill a Mockingbird', 'Harper Lee', 'Fiction', 281)," +
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"(3, '1984', 'George Orwell', 'Dystopian', 328)," +
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"(4, 'Pride and Prejudice', 'Jane Austen', 'Romance', 432)," +
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"(5, 'The Catcher in the Rye', 'J.D. Salinger', 'Fiction', 277);" +
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"CREATE TABLE reading_sessions (" +
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" session_id INT PRIMARY KEY," +
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" book_id INT," +
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" reader_name VARCHAR(100)," +
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" pages_read INT," +
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" session_rating INT," +
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" FOREIGN KEY (book_id) REFERENCES books(book_id)" +
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");" +
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"INSERT INTO reading_sessions (session_id, book_id, " +
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"reader_name, pages_read, session_rating) VALUES" +
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"(1, 1, 'Alice', 50, 5)," +
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"(2, 1, 'Bob', 60, 1)," +
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"(3, 1, 'Carol', 40, 4)," +
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"(4, 1, 'David', 30, 2)," +
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"(5, 1, 'Emma', 45, 5)," +
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"(6, 2, 'Frank', 80, 4)," +
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"(7, 2, 'Grace', 70, 4)," +
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"(8, 2, 'Henry', 90, 5)," +
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"(9, 2, 'Ivy', 60, 4)," +
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"(10, 2, 'Jack', 75, 4)," +
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"(11, 3, 'Kate', 100, 2)," +
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"(12, 3, 'Liam', 120, 1)," +
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"(13, 3, 'Mia', 80, 2)," +
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"(14, 3, 'Noah', 90, 1)," +
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"(15, 3, 'Olivia', 110, 4)," +
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"(16, 3, 'Paul', 95, 5)," +
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"(17, 4, 'Quinn', 150, 3)," +
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"(18, 4, 'Ruby', 140, 3)," +
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"(19, 5, 'Sam', 80, 1)," +
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"(20, 5, 'Tara', 70, 2);"
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),
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],
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)
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internal class MysqlTest {
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@Test
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@Throws(SQLException::class, FileNotFoundException::class)
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fun testScript(@EmbeddedDatabase dataSource: DataSource) {
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dataSource.connection.use { connection ->
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connection.createStatement().use { statement ->
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statement.executeQuery(
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BufferedReader(
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FileReader(
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(
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"src/main/kotlin/g3601_3700/" +
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"s3642_find_books_with_" +
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"polarized_opinions/" +
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"script.sql"
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),
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),
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)
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.lines()
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.collect(Collectors.joining("\n"))
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.replace("#.*?\\r?\\n".toRegex(), ""),
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).use { resultSet ->
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assertThat<Boolean>(resultSet.next(), equalTo<Boolean>(true))
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assertThat<String>(resultSet.getNString(1), equalTo<String>("1"))
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assertThat<String>(
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resultSet.getNString(2),
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equalTo<String>("The Great Gatsby"),
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)
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assertThat<String>(
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resultSet.getNString(3),
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equalTo<String>("F. Scott"),
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)
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assertThat<String>(resultSet.getNString(4), equalTo<String>("Fiction"))
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assertThat<String>(resultSet.getNString(5), equalTo<String>("180"))
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assertThat<String>(resultSet.getNString(6), equalTo<String>("4"))
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assertThat<String>(resultSet.getNString(7), equalTo<String>("1.00"))
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assertThat<Boolean>(resultSet.next(), equalTo<Boolean>(true))
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assertThat<String>(resultSet.getNString(1), equalTo<String>("3"))
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assertThat<String>(resultSet.getNString(2), equalTo<String>("1984"))
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assertThat<String>(
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resultSet.getNString(3),
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equalTo<String>("George Orwell"),
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)
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assertThat<String>(
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resultSet.getNString(4),
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equalTo<String>("Dystopian"),
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)
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assertThat<String>(resultSet.getNString(5), equalTo<String>("328"))
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assertThat<String>(resultSet.getNString(6), equalTo<String>("4"))
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assertThat<String>(resultSet.getNString(7), equalTo<String>("1.00"))
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assertThat<Boolean>(resultSet.next(), equalTo<Boolean>(false))
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}
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}
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}
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}
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}

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