SQL: First and Last Event Per User

Business Context

NewWave Telecom & Technologies uses customer activity events to analyze engagement across its digital services. The Data Analyst team needs a per-user lifecycle summary for the NewWave customer activity dashboard.

Task

Write a PostgreSQL query that returns the first and last valid event for every user in users, including users who have no valid events.

Requirements

1. Exclude events whose event_at is NULL.
2. Rank events independently for each user by timestamp, using event_id as the deterministic tie-breaker.
3. Return the first and last event timestamps and event names.
4. Return elapsed minutes between the first and last event, or NULL when no valid event exists.
5. Preserve users without events, and label unmatched event types as Unknown.
6. Sort by user_id.