SELECT
session_id,
session_set_id,
session_set_peak
FROM
peak_sessions
JOIN (
SELECT
session_id,
peak_sessions_sets.id as session_set_id,
MAX(peak) as session_set_peak
FROM
peak_sessions_sets
JOIN peak_sessions ON peak_sessions_sets.session_id = peak_sessions.id
GROUP BY session_id
) as max_peaks ON max_peaks.session_id = peak_sessions.id
WHERE peak_sessions.user_id = 1