MySQL 8.0

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CREATE TABLE `comments`
    (`id` int auto_increment primary key,
     `user_id` int, 
     `text` varchar(20), 
     `created_at` timestamp
    );
    
INSERT INTO `comments`
    (`user_id`, `text`, `created_at`)
VALUES
    (28, 'comment text', '2021-08-07 17:00:52'),
    (28, 'comment text', '2021-08-12 10:07:11'),
    (28, 'comment text', '2021-08-12 10:08:14'),
    (28, 'comment text', '2021-08-12 10:14:08'),
    (28, 'comment text', '2021-08-13 20:32:21'),
    (28, 'comment text', '2021-08-13 20:34:17'),
    (28, 'comment text', '2021-08-13 21:55:09');
SELECT COUNT(c.id) count
FROM comments c
WHERE c.user_id = '28' 
  AND COALESCE(
        TIMESTAMPDIFF(
          SECOND, 
          (
            SELECT MAX(cc.created_at)
            FROM comments cc
            WHERE cc.user_id = c.user_id AND cc.created_at < c.created_at
          ), 
          c.created_at
        ), 
        3601
      ) > 3600 


count


4


WITH cte AS (
  SELECT *, 
         TIMESTAMPDIFF(
          SECOND,
          LAG(created_at) OVER (PARTITION BY user_id ORDER BY created_at),
          created_at
        ) diff
  FROM comments
)
SELECT COUNT(id) count
FROM cte
WHERE user_id = '28' AND (diff IS NULL OR diff > 3600) 


count


4