MySQL 8.0

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CREATE TABLE hotels (
    id INT NOT NULL PRIMARY KEY,
    name VARCHAR(11)
);
INSERT INTO hotels (id, name) VALUES
    (1, 'Hotel 1');
CREATE TABLE rooms (
    id INT NOT NULL PRIMARY KEY,
    hotel_id INT,
    name VARCHAR(100),
    number INT,
    available_from TIME,
    available_to TIME
);
INSERT INTO rooms (id, hotel_id, name, number, available_from, available_to) VALUES
    (1, 1, 'Single', 528, '09:00:00', '21:00:00');
CREATE TABLE reservations (
    id INT NOT NULL PRIMARY KEY,
    hotel_id INT,
    room_id INT,
    `start` DATETIME,
    `end` DATETIME, STATUS VARCHAR(30)
);
INSERT INTO reservations(id, hotel_id, room_id, `start`, `end`, STATUS) VALUES
    (1, 1, 1, '2019-01-03 15:00:00', '2019-01-03 17:00:00', 'Confirmed'),
    (2, 1, 1, '2019-01-03 18:00:00', '2019-01-03 20:00:00', 'Confirmed');
SELECT rooms.id, rooms.name, timeslots.timeslot
FROM rooms
INNER JOIN (
    SELECT CAST('00:00' AS TIME) AS timeslot UNION
    SELECT CAST('01:00' AS TIME) UNION
    SELECT CAST('02:00' AS TIME) UNION
    SELECT CAST('03:00' AS TIME) UNION
    SELECT CAST('04:00' AS TIME) UNION
    SELECT CAST('05:00' AS TIME) UNION
    SELECT CAST('06:00' AS TIME) UNION
    SELECT CAST('07:00' AS TIME) UNION
    SELECT CAST('08:00' AS TIME) UNION
    SELECT CAST('09:00' AS TIME) UNION
    SELECT CAST('10:00' AS TIME) UNION
    SELECT CAST('11:00' AS TIME) UNION
    SELECT CAST('12:00' AS TIME) UNION
    SELECT CAST('13:00' AS TIME) UNION
    SELECT CAST('14:00' AS TIME) UNION
    SELECT CAST('15:00' AS TIME) UNION
    SELECT CAST('16:00' AS TIME) UNION
    SELECT CAST('17:00' AS TIME) UNION
    SELECT CAST('18:00' AS TIME) UNION
    SELECT CAST('19:00' AS TIME) UNION
    SELECT CAST('20:00' AS TIME) UNION
    SELECT CAST('21:00' AS TIME) UNION
    SELECT CAST('22:00' AS TIME) UNION
    SELECT CAST('23:00' AS TIME)
) AS timeslots ON timeslots.timeslot >= rooms.Available_From AND timeslots.timeslot <= rooms.Available_To
WHERE NOT EXISTS (
    SELECT 1
    FROM reservations
    WHERE room_id = rooms.id
    AND TIMESTAMP('2019-01-03', timeslots.timeslot) >= `start` AND TIMESTAMP('2019-01-03', timeslots.timeslot) <= `end`
)


id
name
timeslot


1
Single
09:00:00

1
Single
10:00:00

1
Single
11:00:00

1
Single
12:00:00

1
Single
13:00:00

1
Single
14:00:00

1
Single
21:00:00
id	name	timeslot
1	Single	09:00:00
1	Single	10:00:00
1	Single	11:00:00
1	Single	12:00:00
1	Single	13:00:00
1	Single	14:00:00
1	Single	21:00:00