WITH START AS (SELECT user_id, TIMESTAMP, ROW_NUMBER() OVER(partition BY user_id ORDER BY TIMESTAMP DESC) AS INDEX
FROM session_start
ORDER BY user_id, INDEX)

WITH END AS (SELECT user_id, TIMESTAMP, ROW_NUMBER() OVER(partition BY user_id ORDER BY TIMESTAMP DESC) AS INDEX
FROM session_end
ORDER BY user_id, INDEX)

WITH sessions AS SELECT START.user_id AS user_id, START.INDEX AS INDEX, datediff(SECOND, START.TIMESTAMP, END.TIMESTAMP) AS duration
FROM START INNER JOIN END ON START.user_id = END.user_id AND START.INDEX = END.INDEX

SELECT user_id, avg(duration) AS avg_seconds
FROM sessions
GROUP BY user_id