MySQL 8.0

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create table working_hours (
  start_time datetime not null, 
  end_time datetime not null
);
insert into working_hours (start_time, end_time) values
('2018-12-29 08:00', '2018-12-29 12:00'),
('2018-12-29 12:30', '2018-12-29 16:00');
create table visits (
  start_time datetime not null primary key, 
  end_time datetime not null, 
  visit_status int not null default 0
);
insert into visits (start_time, end_time, visit_status) values
('2018-12-29 08:00', '2018-12-29 08:30', 1),
('2018-12-29 09:00', '2018-12-29 10:00', 1),
('2018-12-29 10:00', '2018-12-29 10:40', 1),
('2018-12-29 10:40', '2018-12-29 11:10', 2),
('2018-12-29 11:10', '2018-12-29 11:40', 0),
('2018-12-29 12:30', '2018-12-29 13:00', 0),
('2018-12-29 13:00', '2018-12-29 14:00', 0),
('2018-12-29 15:30', '2018-12-29 16:00', 0);
create table nums (num int primary key not null);
insert into nums (num) VALUES 
(00),(01),(02),(03),(04),(05),(06),(07),(08),(09),
(10),(11),(12),(13),(14),(15),(16),(17),(18),(19),
(20),(21),(22),(23),(24),(25),(26),(27),(28),(29),
(30),(31),(32),(33),(34),(35),(36),(37),(38),(39),
(40),(41),(42),(43),(44),(45),(46),(47),(48),(49);
SELECT DISTINCT 
 CAST(gaps.start_dt + INTERVAL (nums.num * 30) MINUTE AS TIME) as start_time
FROM
(
  SELECT rnk, MIN(prev_dt) as start_dt, MIN(start_dt) as end_dt
  FROM
  (
    SELECT 
    start_time AS start_dt, 
    end_time as end_dt, 
    @prev_dt as prev_dt,
    (CASE
     WHEN @prev_dt = start_time AND @prev_dt := end_time THEN @rnk
     WHEN @prev_dt := end_time THEN @rnk := @rnk + 1
     END) AS rnk
    FROM visits
    CROSS JOIN (SELECT @prev_dt := null, @rnk := 0) vars
    WHERE visit_status <> 2
    ORDER BY start_time
  ) AS vst
  GROUP BY rnk
  HAVING CAST(MIN(start_dt) AS DATE) = CAST(MIN(prev_dt) AS DATE)
     AND TIMEDIFF(MIN(start_dt), MIN(prev_dt)) >= CAST('00:30' AS TIME)
) gaps
JOIN working_hours wrk 
  ON wrk.start_time <= gaps.start_dt AND wrk.end_time >= gaps.end_dt
JOIN nums
  ON nums.num BETWEEN 0 AND 47
 AND gaps.start_dt + INTERVAL (nums.num * 30) MINUTE < gaps.end_dt;


start_time


14:00:00

10:40:00

08:30:00

14:30:00

15:00:00