Sure, let's break down the solution to the problem step-by-step:
1. Extract Date and Unique Users:
- Extract the date from the DATETIME.
- For each date-product combination, count the number of unique users.
2. Filter Date-Product Combinations with at Least 5 Users:
- We need to focus only on those date-product combinations where there are at least 5 unique users.
3. Calculate Retention:
- For each valid date-product combination, determine if users re-use the same product within 7 days (not including the same day).
- Calculate the total retained users.
4. Determine the Date-Product Combination with Maximum Retention:
- Identify the combination with the highest number of retained users.
Since I need to provide a MySQL solution, here is the step-by-step MySQL syntax to achieve the solution:
```sql
WITH activity_dates AS (
SELECT
USER_ID,
PRODUCT,
DATE(DATETIME) AS DATE
FROM
ACTIVITY
),
valid_combinations AS (
SELECT
DATE,
PRODUCT,
COUNT(DISTINCT USER_ID) AS total_users_this_date
FROM
activity_dates
GROUP BY
DATE,
PRODUCT
HAVING
COUNT(DISTINCT USER_ID) >= 5
),
retention AS (
SELECT
ad1.DATE AS start_date,
ad1.PRODUCT,
COUNT(DISTINCT ad1.USER_ID) AS total_users_this_date,
COUNT(DISTINCT ad1.USER_ID) AS total_retained_users
FROM
activity_dates ad1
JOIN
activity_dates ad2
ON
ad1.USER_ID = ad2.USER_ID
AND ad1.PRODUCT = ad2.PRODUCT
AND ad2.DATE > ad1.DATE
AND ad2.DATE <= DATE_ADD(ad1.DATE, INTERVAL 7 DAY)
AND ad2.DATE != ad1.DATE
WHERE
(ad1.DATE, ad1.PRODUCT) IN (
SELECT
DATE, PRODUCT
FROM
valid_combinations
)
GROUP BY
ad1.DATE,
ad1.PRODUCT
)
SELECT
start_date AS date,
PRODUCT AS product,
total_users_this_date,
total_retained_users
FROM
retention
ORDER BY
total_retained_users DESC
LIMIT 1;
```
### Explanation:
1. activity_dates:
- This common table expression (CTE) extracts the distinct `USER_ID`, `PRODUCT`, and `DATE` from the `ACTIVITY` table.
2. valid_combinations:
- This CTE calculates the number of unique users for each `DATE` and `PRODUCT` combination and filters those combinations with at least 5 users.
3. retention:
- This CTE joins the `activity_dates` table with itself to check for users who reuse the same product within 7 days (excluding the same day). It then groups by the initial `DATE` and `PRODUCT` to count the total users and those retained.
4. Final Selection:
- The main query selects the result from the `retention` CTE, sorting by the number of retained users in descending order and limiting the output to the top row.
By running this SQL query, you should get the date-product combination with the maximum retention based on the given conditions.
