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‎src/main/kotlin/g3701_3800/s3716_find_churn_risk_customers/readme.md‎

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3716\. Find Churn Risk Customers
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Medium
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Table: `subscription_events`
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+------------------+---------+
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| Column Name | Type |
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+------------------+---------+
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| event_id | int |
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| user_id | int |
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| event_date | date |
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| event_type | varchar |
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| plan_name | varchar |
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| monthly_amount | decimal |
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+------------------+---------+
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event_id is the unique identifier for this table.
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event_type can be start, upgrade, downgrade, or cancel.
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plan_name can be basic, standard, premium, or NULL (when event_type is cancel).
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monthly_amount represents the monthly subscription cost after this event.
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For cancel events, monthly_amount is 0.
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Write a solution to **Find Churn Risk Customers** - users who show warning signs before churning. A user is considered **churn risk customer** if they meet ALL the following criteria:
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* Currently have an **active subscription** (their last event is not cancel).
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* Have performed **at least one** downgrade in their subscription history.
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* Their **current plan revenue** is less than `50%` of their historical maximum plan revenue.
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* Have been a subscriber for **at least** `60` days.
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Return _the result table ordered by_ `days_as_subscriber` _in **descending** order, then by_ `user_id` _in **ascending** 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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subscription\_events table:
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+----------+---------+------------+------------+-----------+----------------+
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| event_id | user_id | event_date | event_type | plan_name | monthly_amount |
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+----------+---------+------------+------------+-----------+----------------+
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| 1 | 501 | 2024-01-01 | start | premium | 29.99 |
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| 2 | 501 | 2024-02-15 | downgrade | standard | 19.99 |
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| 3 | 501 | 2024-03-20 | downgrade | basic | 9.99 |
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| 4 | 502 | 2024-01-05 | start | standard | 19.99 |
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| 5 | 502 | 2024-02-10 | upgrade | premium | 29.99 |
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| 6 | 502 | 2024-03-15 | downgrade | basic | 9.99 |
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| 7 | 503 | 2024-01-10 | start | basic | 9.99 |
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| 8 | 503 | 2024-02-20 | upgrade | standard | 19.99 |
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| 9 | 503 | 2024-03-25 | upgrade | premium | 29.99 |
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| 10 | 504 | 2024-01-15 | start | premium | 29.99 |
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| 11 | 504 | 2024-03-01 | downgrade | standard | 19.99 |
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| 12 | 504 | 2024-03-30 | cancel | NULL | 0.00 |
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| 13 | 505 | 2024-02-01 | start | basic | 9.99 |
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| 14 | 505 | 2024-02-28 | upgrade | standard | 19.99 |
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| 15 | 506 | 2024-01-20 | start | premium | 29.99 |
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| 16 | 506 | 2024-03-10 | downgrade | basic | 9.99 |
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+----------+---------+------------+------------+-----------+----------------+
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**Output:**
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+---------+--------------+------------------------+-----------------------+--------------------+
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| user_id | current_plan | current_monthly_amount | max_historical_amount | days_as_subscriber |
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+---------+--------------+------------------------+-----------------------+--------------------+
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| 501 | basic | 9.99 | 29.99 | 79 |
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| 502 | basic | 9.99 | 29.99 | 69 |
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+---------+--------------+------------------------+-----------------------+--------------------+
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**Explanation:**
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* **User 501**:
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* Currently active: Last event is downgrade to basic (not cancelled)
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* Has downgrades: Yes, 2 downgrades in history
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* Current revenue (9.99) vs max (29.99): 9.99/29.99 = 33.3% (less than 50%)
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* Days as subscriber: Jan 1 to Mar 20 = 79 days (at least 60)
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* Result: **Churn Risk Customer**
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* **User 502**:
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* Currently active: Last event is downgrade to basic (not cancelled)
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* Has downgrades: Yes, 1 downgrade in history
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* Current revenue (9.99) vs max (29.99): 9.99/29.99 = 33.3% (less than 50%)
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* Days as subscriber: Jan 5 to Mar 15 = 70 days (at least 60)
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* Result: **Churn Risk Customer**
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* **User 503**:
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* Currently active: Last event is upgrade to premium (not cancelled)
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* Has downgrades: No downgrades in history
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* Result: **Not at-risk** (no downgrade history)
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* **User 504**:
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* Currently active: Last event is cancel
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* Result: **Not at-risk** (subscription cancelled)
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* **User 505**:
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* Currently active: Last event is 'upgrade' to standard (not cancelled)
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* Has downgrades: No downgrades in history
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* Result: **Not at-risk** (no downgrade history)
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* **User 506**:
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* Currently active: Last event is downgrade to basic (not cancelled)
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* Has downgrades: Yes, 1 downgrade in history
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* Current revenue (9.99) vs max (29.99): 9.99/29.99 = 33.3% (less than 50%)
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* Days as subscriber: Jan 20 to Mar 10 = 50 days (less than 60)
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* Result: **Not at-risk** (insufficient subscription duration)
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Result table is ordered by days\_as\_subscriber DESC, then user\_id ASC.
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**Note:** days\_as\_subscriber is calculated from the first event date to the last event date for each user.

‎src/main/kotlin/g3701_3800/s3716_find_churn_risk_customers/script.sql‎

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# Write your MySQL query statement below
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# #Medium #Database #2025_10_16_Time_256_ms_(96.87%)_Space_0.0_MB_(100.00%)
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WITH UserStats AS (
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SELECT
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user_id,
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MIN(CASE WHEN event_type = 'start' THEN event_date END) AS start_date,
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MAX(event_date) AS last_event,
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MAX(monthly_amount) AS max_revenue,
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SUM(CASE WHEN event_type = 'downgrade' THEN 1 ELSE 0 END) AS downgrade_count
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FROM subscription_events
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GROUP BY user_id
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)
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SELECT
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us.user_id,
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se.plan_name AS current_plan,
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se.monthly_amount AS current_monthly_amount,
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us.max_revenue AS max_historical_amount,
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TIMESTAMPDIFF(DAY, us.start_date, us.last_event) AS days_as_subscriber
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FROM UserStats us
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JOIN subscription_events se
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ON us.user_id = se.user_id
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AND us.last_event = se.event_date
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WHERE se.event_type <> 'cancel'
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AND us.downgrade_count > 0
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AND se.monthly_amount * 2 < us.max_revenue
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AND TIMESTAMPDIFF(DAY, us.start_date, us.last_event) > 60
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ORDER BY
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TIMESTAMPDIFF(DAY, us.start_date, us.last_event) DESC,
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us.user_id ASC;

‎src/test/kotlin/g3701_3800/s3716_find_churn_risk_customers/MysqlTest.kt‎

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package g3701_3800.s3716_find_churn_risk_customers
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import org.hamcrest.CoreMatchers
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import org.hamcrest.MatcherAssert
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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 subscription_events (" +
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" event_id INTEGER PRIMARY KEY," +
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" user_id INTEGER NOT NULL," +
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" event_date DATE NOT NULL," +
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" event_type VARCHAR(20) NOT NULL," +
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" plan_name VARCHAR(20)," +
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" monthly_amount DECIMAL(10,2) NOT NULL" +
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");" +
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"INSERT INTO subscription_events (event_id, user_id, event_date, " +
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"event_type, plan_name, monthly_amount) VALUES" +
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"(1, 501, '2024-01-01', 'start', 'premium', 29.99)," +
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"(2, 501, '2024-02-15', 'downgrade', 'standard', 19.99)," +
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"(3, 501, '2024-03-20', 'downgrade', 'basic', 9.99)," +
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"(4, 502, '2024-01-05', 'start', 'standard', 19.99)," +
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"(5, 502, '2024-02-10', 'upgrade', 'premium', 29.99)," +
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"(6, 502, '2024-03-15', 'downgrade', 'basic', 9.99)," +
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"(7, 503, '2024-01-10', 'start', 'basic', 9.99)," +
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"(8, 503, '2024-02-20', 'upgrade', 'standard', 19.99)," +
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"(9, 503, '2024-03-25', 'upgrade', 'premium', 29.99)," +
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"(10, 504, '2024-01-15', 'start', 'premium', 29.99)," +
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"(11, 504, '2024-03-01', 'downgrade','standard', 19.99)," +
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"(12, 504, '2024-03-30', 'cancel', NULL, 0.00)," +
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"(13, 505, '2024-02-01', 'start', 'basic', 9.99)," +
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"(14, 505, '2024-02-28', 'upgrade', 'standard', 19.99)," +
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"(15, 506, '2024-01-20', 'start', 'premium', 29.99)," +
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"(16, 506, '2024-03-10', 'downgrade','basic', 9.99);" +
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""
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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/g3701_3800/" +
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"s3716_find_churn_risk_customers/" +
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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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MatcherAssert.assertThat<Boolean>(resultSet.next(), CoreMatchers.equalTo<Boolean>(true))
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MatcherAssert.assertThat<String>(resultSet.getString(1), CoreMatchers.equalTo<String>("501"))
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MatcherAssert.assertThat<String>(resultSet.getString(2), CoreMatchers.equalTo<String>("basic"))
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MatcherAssert.assertThat<String>(resultSet.getString(3), CoreMatchers.equalTo<String>("9.99"))
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MatcherAssert.assertThat<String>(resultSet.getString(4), CoreMatchers.equalTo<String>("29.99"))
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MatcherAssert.assertThat<String>(resultSet.getString(5), CoreMatchers.equalTo<String>("79"))
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MatcherAssert.assertThat<Boolean>(resultSet.next(), CoreMatchers.equalTo<Boolean>(true))
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MatcherAssert.assertThat<String>(resultSet.getString(1), CoreMatchers.equalTo<String>("502"))
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MatcherAssert.assertThat<String>(resultSet.getString(2), CoreMatchers.equalTo<String>("basic"))
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MatcherAssert.assertThat<String>(resultSet.getString(3), CoreMatchers.equalTo<String>("9.99"))
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MatcherAssert.assertThat<String>(resultSet.getString(4), CoreMatchers.equalTo<String>("29.99"))
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MatcherAssert.assertThat<String>(resultSet.getString(5), CoreMatchers.equalTo<String>("70"))
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MatcherAssert.assertThat<Boolean>(resultSet.next(), CoreMatchers.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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