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How to calculate an SQL MAX() for any N-second duration within a certain timeframe
-- Timestamped log of users viewing items
CREATE TABLE user_item_views (
  user_id integer,
  item_id integer,
  timestamp timestamp
);

User ID  Max items viewed in N seconds, between START and END.
...      ...
...      ...
...      ...

SELECT
  s.timestamp,
  v.user_id,
  (
   SELECT COUNT(*) FROM user_item_views
    WHERE timestamp BETWEEN s.timestamp AND ADDTIME(s.timestamp, '00:00:59')
          AND user_id = v.user_id
  ) item_count
FROM
  every_second s
  LEFT JOIN user_item_views v ON v.timestamp = s.timestamp
GROUP BY
  s.timestamp,
  v.user_id

SELECT
      user_id
    , MAX(max_count) AS max_count
FROM
  ( SELECT
          a.user_id
        , COUNT(*)      AS max_count
    FROM
          user_item_views AS a
      JOIN
          user_item_views AS b
        ON  a.user_id = b.user_id
        AND a.timestamp <= b.timestamp
        AND b.timestamp < a.timestamp + INTERVAL 60 SECOND
    GROUP BY
          a.user_id
        , a.timestamp
  ) AS grp
GROUP BY
      user_id