Rolling 30-Day Active Users

BCG wants to monitor engagement across its internal digital products and analytics surfaces. Using a raw event log, calculate the number of unique users active during the trailing 30 calendar days for each date in the event range.

Write a PostgreSQL query that produces one row per calendar date from the minimum through maximum event date.

Requirements

1. Count each non-null user_id only once within each 30-day window.
2. Include the current date and the preceding 29 calendar days in each window.
3. Include dates with no events between the minimum and maximum event dates.
4. Sort the output chronologically by date.