SQLite 3.27

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CREATE TABLE tablename (`id` INTEGER, `datetime` TEXT);
✓
INSERT INTO tablename (`id`, `datetime`) VALUES
  ('1', '2020-12-26 19:08:49'),
  ('2', '2020-12-26 19:08:50'),
  ('3', '2020-12-26 19:08:51'),
  ('4', '2020-12-26 19:08:51'),
  ('5', '2020-12-26 19:09:07'),
  ('6', '2020-12-26 19:11:45'),
  ('7', '2020-12-26 19:52:49'),
  ('8', '2020-12-26 19:52:50');
✓
SELECT *
FROM (
  SELECT id,
         strftime('%s', datetime) - strftime('%s', FIRST_VALUE(datetime) OVER (ORDER BY id)) AS diff_in_secs
  FROM tablename
)
WHERE id > 1


id
diff_in_secs


2
1

3
2

4
2

5
18

6
176

7
2640

8
2641


SELECT t.id, 
       strftime('%s', t.datetime) - strftime('%s', m.datetime) AS diff_in_secs
FROM tablename t
INNER JOIN (SELECT * FROM tablename ORDER BY id LIMIT 1) m
ON m.id < t.id


id
diff_in_secs


2
1

3
2

4
2

5
18

6
176

7
2640

8
2641