MySQL 8.0

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select version();


version()


8.0.27


create table agenda (PERID INT,	DAYNUM INt,START_HOUR time, END_HOUR time);
insert into agenda VALUES (100,	3, '08:00', '11:00');
insert into agenda VALUES (100,	3, '13:00', '15:00');
insert into agenda VALUES (100,	3, '16:00', '18:00');
select * from agenda


PERID
DAYNUM
START_HOUR
END_HOUR


100
3
08:00:00
11:00:00

100
3
13:00:00
15:00:00

100
3
16:00:00
18:00:00


select perid, daynum, start_hour, end_hour, 
    case 
      when (LEAD(start_hour) OVER(order by perid, daynum, start_hour, end_hour)) > end_hour then end_hour
      else NULL
    end as START_H,
    case 
      when (LEAD(start_hour) OVER(order by perid, daynum, start_hour, end_hour)) > end_hour then (LEAD(start_hour) OVER(order by perid, daynum, start_hour, end_hour))
      else NULL
    end as END_H
from agenda


perid
daynum
start_hour
end_hour
START_H
END_H


100
3
08:00:00
11:00:00
11:00:00
13:00:00

100
3
13:00:00
15:00:00
15:00:00
16:00:00

100
3
16:00:00
18:00:00
null
null


select distinct START_H, end_h
from
(
select perid, daynum, start_hour, end_hour, 
    case 
      when (LEAD(start_hour) OVER(order by perid, daynum, start_hour, end_hour)) > end_hour then end_hour
      else NULL
    end as START_H,
    case 
      when (LEAD(start_hour) OVER(order by perid, daynum, start_hour, end_hour)) > end_hour then (LEAD(start_hour) OVER(order by perid, daynum, start_hour, end_hour))
      else NULL
    end as END_H
from agenda
)as sq
WHERE start_h IS NOT NULL 
   and end_h IS NOT NULL


START_H
end_h


11:00:00
13:00:00

15:00:00
16:00:00

PERID	DAYNUM	START_HOUR	END_HOUR
100	3	08:00:00	11:00:00
100	3	13:00:00	15:00:00
100	3	16:00:00	18:00:00