Calculate Average Camera Switches per Session

Given the PostgreSQL table: ```sql camera_events ( event_id BIGINT PRIMARY KEY, session_id BIGINT NOT NULL, event_ts TIMESTAMP NOT NULL, camera_position TEXT NOT NULL CHECK (camera_position IN ('front', 'rear')) ) ``` A camera switch occurs when an event's `camera_position` differs from the immediately preceding camera event in the same session. The first event in a session is not a switch. Write one read-only `SELECT`/CTE query that returns the average number of camera switches per session across all sessions represented in the table. Return exactly one row and one column: ```text average_switches_per_session ``` ### Constraints and Clarifications - Order events within a session by `event_ts`, then `event_id` to break ties. - Repeated consecutive records of the same position do not count as switches. - Sessions with one event contribute zero switches and must remain in the denominator. - Return a numeric result, not integer division; an empty table may return `NULL`. ```hint Compare adjacent rows Use a window function to expose the preceding position, turn each comparison into zero or one, aggregate per session, and then average. ``` ### Evaluation Focus - Correct use of `LAG` within each session. - Inclusion of zero-switch sessions. - Deterministic ordering and a numeric average. - A single read-only statement with the exact output alias. ### Extension How would you compute the distribution of switch counts rather than only the average?