MySQL 8.0

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select @@version;


@@version


8.0.30


CREATE TABLE Stack (
  `id` INTEGER,
  `datetime` DATETIME,
  `status` VARCHAR(3)
);
INSERT INTO Stack
  (`id`, `datetime`, `status`)
VALUES
  ('1', '2020-01-01 10:00:00', 'ON'),
  ('2', '2020-01-01 11:00:00', 'ON'),
  ('3', '2020-01-01 12:00:00', 'OFF'),
  ('4', '2020-01-01 12:30:00', 'OFF'),
  ('5', '2020-01-01 15:00:00', 'ON'),
  ('6', '2020-01-01 16:00:00', 'ON'),
  ('7', '2020-01-01 17:30:00', 'OFF'),
  ('8', '2020-01-01 18:00:00', 'ON');

Records: 8  Duplicates: 0  Warnings: 0


select
  sec_to_time(sum(case when status = 'OFF' then secs else -secs end))
from
(
  select
    timestampdiff(second, date '1990-01-01', datetime) as secs,
    status, 
    lag(status) over (order by datetime) as prev_status
  from 
  (
    select datetime, status from Stack
    union all
    select now(), 'OFF'
  ) end_row_added
) with_previous_status
where status <> prev_status or prev_status is null;


sec_to_time(sum(case when status = 'OFF' then secs else -secs end))


838:59:59