Просто присоединитесь к таблице доступности дважды
SELECT rooms.* FROM rooms, availability as a1, availability as a2
WHERE rooms.id = 123
AND a1.room_id = rooms.id
AND a2.room_id= rooms.id
AND a1.date_occupied + 1 = a2.date_occupied
или, если мы не собираемся писать SQL, как его 1985:
SELECT rooms.* FROM rooms
JOIN availability a1 on a1.room_id = rooms.id
Join availability a2 on a2.room_id = rooms.id AND a1.date_occupied + 1 = a2.date_occupied
WHERE rooms.id = 123