Write a SQL query to find the monthly retention rate of users who signed up for a free tier of MongoDB Atlas over the last year.
Treat retention for each calendar month as the percentage of eligible free-tier users who had activity during that month. Include users from the trailing 12-month signup window and exclude paid-tier users.
month_start, eligible_users, retained_users, and retention_rate.month_start; calculate the rate as a percentage rounded to two decimal places.| Column | Type | Description |
|---|---|---|
| user_idPK | INT | Unique Atlas user identifier |
| signup_date | DATE | Date the user signed up |
| tier | VARCHAR(20) | User subscription tier |
| Column | Type | Description |
|---|---|---|
| activity_idPK | INT | Unique activity event identifier |
| user_id | INT | User associated with the activity |
| activity_date | DATE | Date of the Atlas activity |
| activity_type | VARCHAR(30) | Type of product activity |