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

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

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

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