Added task 3716

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

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+3716\. Find Churn Risk Customers
+
+Medium
+
+Table: `subscription_events`
+
+    +------------------+---------+
+    | Column Name      | Type    |
+    +------------------+---------+
+    | event_id         | int     |
+    | user_id          | int     |
+    | event_date       | date    |
+    | event_type       | varchar |
+    | plan_name        | varchar |
+    | monthly_amount   | decimal |
+    +------------------+---------+
+    event_id is the unique identifier for this table.
+    event_type can be start, upgrade, downgrade, or cancel.
+    plan_name can be basic, standard, premium, or NULL (when event_type is cancel).
+    monthly_amount represents the monthly subscription cost after this event.
+    For cancel events, monthly_amount is 0. 
+
+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:
+
+*   Currently have an **active subscription** (their last event is not cancel).
+*   Have performed **at least one** downgrade in their subscription history.
+*   Their **current plan revenue** is less than `50%` of their historical maximum plan revenue.
+*   Have been a subscriber for **at least** `60` days.
+
+Return _the result table ordered by_ `days_as_subscriber` _in **descending** order, then by_ `user_id` _in **ascending** order_.
+
+The result format is in the following example.
+
+**Example:**
+
+**Input:**
+
+subscription\_events table:
+
+    +----------+---------+------------+------------+-----------+----------------+
+    | event_id | user_id | event_date | event_type | plan_name | monthly_amount |
+    +----------+---------+------------+------------+-----------+----------------+
+    | 1        | 501     | 2024-01-01 | start      | premium   | 29.99          |
+    | 2        | 501     | 2024-02-15 | downgrade  | standard  | 19.99          |
+    | 3        | 501     | 2024-03-20 | downgrade  | basic     | 9.99           |
+    | 4        | 502     | 2024-01-05 | start      | standard  | 19.99          |
+    | 5        | 502     | 2024-02-10 | upgrade    | premium   | 29.99          |
+    | 6        | 502     | 2024-03-15 | downgrade  | basic     | 9.99           |
+    | 7        | 503     | 2024-01-10 | start      | basic     | 9.99           |
+    | 8        | 503     | 2024-02-20 | upgrade    | standard  | 19.99          |
+    | 9        | 503     | 2024-03-25 | upgrade    | premium   | 29.99          |
+    | 10       | 504     | 2024-01-15 | start      | premium   | 29.99          |
+    | 11       | 504     | 2024-03-01 | downgrade  | standard  | 19.99          |
+    | 12       | 504     | 2024-03-30 | cancel     | NULL      | 0.00           |
+    | 13       | 505     | 2024-02-01 | start      | basic     | 9.99           |
+    | 14       | 505     | 2024-02-28 | upgrade    | standard  | 19.99          |
+    | 15       | 506     | 2024-01-20 | start      | premium   | 29.99          |
+    | 16       | 506     | 2024-03-10 | downgrade  | basic     | 9.99           |
+    +----------+---------+------------+------------+-----------+----------------+
+
+**Output:**
+
+    +---------+--------------+------------------------+-----------------------+--------------------+
+    | user_id | current_plan | current_monthly_amount | max_historical_amount | days_as_subscriber |
+    +---------+--------------+------------------------+-----------------------+--------------------+
+    | 501     | basic        | 9.99                   | 29.99                 | 79                 |
+    | 502     | basic        | 9.99                   | 29.99                 | 69                 |
+    +---------+--------------+------------------------+-----------------------+--------------------+
+
+**Explanation:**
+
+*   **User 501**:
+    *   Currently active: Last event is downgrade to basic (not cancelled)
+    *   Has downgrades: Yes, 2 downgrades in history
+    *   Current revenue (9.99) vs max (29.99): 9.99/29.99 = 33.3% (less than 50%)
+    *   Days as subscriber: Jan 1 to Mar 20 = 79 days (at least 60)
+    *   Result: **Churn Risk Customer**
+*   **User 502**:
+    *   Currently active: Last event is downgrade to basic (not cancelled)
+    *   Has downgrades: Yes, 1 downgrade in history
+    *   Current revenue (9.99) vs max (29.99): 9.99/29.99 = 33.3% (less than 50%)
+    *   Days as subscriber: Jan 5 to Mar 15 = 70 days (at least 60)
+    *   Result: **Churn Risk Customer**
+*   **User 503**:
+    *   Currently active: Last event is upgrade to premium (not cancelled)
+    *   Has downgrades: No downgrades in history
+    *   Result: **Not at-risk** (no downgrade history)
+*   **User 504**:
+    *   Currently active: Last event is cancel
+    *   Result: **Not at-risk** (subscription cancelled)
+*   **User 505**:
+    *   Currently active: Last event is 'upgrade' to standard (not cancelled)
+    *   Has downgrades: No downgrades in history
+    *   Result: **Not at-risk** (no downgrade history)
+*   **User 506**:
+    *   Currently active: Last event is downgrade to basic (not cancelled)
+    *   Has downgrades: Yes, 1 downgrade in history
+    *   Current revenue (9.99) vs max (29.99): 9.99/29.99 = 33.3% (less than 50%)
+    *   Days as subscriber: Jan 20 to Mar 10 = 50 days (less than 60)
+    *   Result: **Not at-risk** (insufficient subscription duration)
+
+Result table is ordered by days\_as\_subscriber DESC, then user\_id ASC.
+
+**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
+# #Medium #Database #2025_10_16_Time_256_ms_(96.87%)_Space_0.0_MB_(100.00%)
+WITH UserStats AS (
+    SELECT 
+        user_id, 
+        MIN(CASE WHEN event_type = 'start' THEN event_date END) AS start_date,
+        MAX(event_date) AS last_event,
+        MAX(monthly_amount) AS max_revenue,
+        SUM(CASE WHEN event_type = 'downgrade' THEN 1 ELSE 0 END) AS downgrade_count
+    FROM subscription_events
+    GROUP BY user_id
+)
+SELECT
+    us.user_id,
+    se.plan_name AS current_plan,
+    se.monthly_amount AS current_monthly_amount,
+    us.max_revenue AS max_historical_amount,
+    TIMESTAMPDIFF(DAY, us.start_date, us.last_event) AS days_as_subscriber
+FROM UserStats us
+JOIN subscription_events se
+    ON us.user_id = se.user_id
+   AND us.last_event = se.event_date
+WHERE se.event_type <> 'cancel'
+  AND us.downgrade_count > 0
+  AND se.monthly_amount * 2 < us.max_revenue
+  AND TIMESTAMPDIFF(DAY, us.start_date, us.last_event) > 60
+ORDER BY
+    TIMESTAMPDIFF(DAY, us.start_date, us.last_event) DESC,
+    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
+
+import org.hamcrest.CoreMatchers
+import org.hamcrest.MatcherAssert
+import org.junit.jupiter.api.Test
+import org.zapodot.junit.db.annotations.EmbeddedDatabase
+import org.zapodot.junit.db.annotations.EmbeddedDatabaseTest
+import org.zapodot.junit.db.common.CompatibilityMode
+import java.io.BufferedReader
+import java.io.FileNotFoundException
+import java.io.FileReader
+import java.sql.SQLException
+import java.util.stream.Collectors
+import javax.sql.DataSource
+
+@EmbeddedDatabaseTest(
+    compatibilityMode = CompatibilityMode.MySQL,
+    initialSqls = [
+        (
+            "CREATE TABLE subscription_events (" +
+                "    event_id INTEGER PRIMARY KEY," +
+                "    user_id INTEGER NOT NULL," +
+                "    event_date DATE NOT NULL," +
+                "    event_type VARCHAR(20) NOT NULL," +
+                "    plan_name VARCHAR(20)," +
+                "    monthly_amount DECIMAL(10,2) NOT NULL" +
+                ");" +
+                "INSERT INTO subscription_events (event_id, user_id, event_date, " +
+                "event_type, plan_name, monthly_amount) VALUES" +
+                "(1, 501, '2024-01-01', 'start',     'premium', 29.99)," +
+                "(2, 501, '2024-02-15', 'downgrade', 'standard', 19.99)," +
+                "(3, 501, '2024-03-20', 'downgrade', 'basic', 9.99)," +
+                "(4, 502, '2024-01-05', 'start',     'standard', 19.99)," +
+                "(5, 502, '2024-02-10', 'upgrade',   'premium', 29.99)," +
+                "(6, 502, '2024-03-15', 'downgrade', 'basic', 9.99)," +
+                "(7, 503, '2024-01-10', 'start',     'basic', 9.99)," +
+                "(8, 503, '2024-02-20', 'upgrade',   'standard', 19.99)," +
+                "(9, 503, '2024-03-25', 'upgrade',   'premium', 29.99)," +
+                "(10, 504, '2024-01-15', 'start',    'premium', 29.99)," +
+                "(11, 504, '2024-03-01', 'downgrade','standard', 19.99)," +
+                "(12, 504, '2024-03-30', 'cancel',   NULL, 0.00)," +
+                "(13, 505, '2024-02-01', 'start',    'basic', 9.99)," +
+                "(14, 505, '2024-02-28', 'upgrade',  'standard', 19.99)," +
+                "(15, 506, '2024-01-20', 'start',    'premium', 29.99)," +
+                "(16, 506, '2024-03-10', 'downgrade','basic', 9.99);" +
+                ""
+            ),
+    ],
+)
+internal class MysqlTest {
+    @Test
+    @Throws(SQLException::class, FileNotFoundException::class)
+    fun testScript(@EmbeddedDatabase dataSource: DataSource) {
+        dataSource.connection.use { connection ->
+            connection.createStatement().use { statement ->
+                statement.executeQuery(
+                    BufferedReader(
+                        FileReader(
+                            (
+                                "src/main/kotlin/g3701_3800/" +
+                                    "s3716_find_churn_risk_customers/" +
+                                    "script.sql"
+                                ),
+                        ),
+                    )
+                        .lines()
+                        .collect(Collectors.joining("\n"))
+                        .replace("#.*?\\r?\\n".toRegex(), ""),
+                ).use { resultSet ->
+                    MatcherAssert.assertThat<Boolean>(resultSet.next(), CoreMatchers.equalTo<Boolean>(true))
+                    MatcherAssert.assertThat<String>(resultSet.getString(1), CoreMatchers.equalTo<String>("501"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(2), CoreMatchers.equalTo<String>("basic"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(3), CoreMatchers.equalTo<String>("9.99"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(4), CoreMatchers.equalTo<String>("29.99"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(5), CoreMatchers.equalTo<String>("79"))
+                    MatcherAssert.assertThat<Boolean>(resultSet.next(), CoreMatchers.equalTo<Boolean>(true))
+                    MatcherAssert.assertThat<String>(resultSet.getString(1), CoreMatchers.equalTo<String>("502"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(2), CoreMatchers.equalTo<String>("basic"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(3), CoreMatchers.equalTo<String>("9.99"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(4), CoreMatchers.equalTo<String>("29.99"))
+                    MatcherAssert.assertThat<String>(resultSet.getString(5), CoreMatchers.equalTo<String>("70"))
+                    MatcherAssert.assertThat<Boolean>(resultSet.next(), CoreMatchers.equalTo<Boolean>(false))
+                }
+            }
+        }
+    }
+}