Su código completo es
SELECT
t1.*
, t2.max_room - `cumulative_room` 'reverse_cumulative room'
FROm (SELECT date,
ant,
num_room,
(@csum:= @csum + num_room) as cumulative_room
from xxx_xxxx_xxxx
WHERE date BETWEEN CURDATE() - INTERVAL 30 DAY AND CURDATE()
AND(select @csum := 0) = 0
order by date) t1
INNER JOIN (SELECT MAX(`cumulative_room`) +1 max_room, `date` FROM (SELECT date,
ant,
num_room,
(@csum:= @csum + num_room) as cumulative_room
from xxx_xxxx_xxxx
WHERE date BETWEEN CURDATE() - INTERVAL 30 DAY AND CURDATE()
AND(select @csum := 0) = 0
order by date) t3 GROUP BY `date`) t2
ON t1.`date` = t2.`date`;
La idea detrás de esto es
db<>fiddle aquí
