if attribute x = "something" do this - mysql

Question

So I have the following tables running, but I'm having a problem on a specific situation.
I have a network of soap dispensers, that I want to keep track of their current soap level. I'm counting the number of pumps (3 mililiters each) and doing greatest(full_capacity - number_pumps * 3, 0) as seen on the View table.
But my problem is: there is table maintenance, and one of the "descriptions" may be "refill". What I wanted was for when a maintenance_description = "refill" for the number_pumps in table records be set to 0 for that exact dispenser. Is is possible? I read about triggers, but couldn't really understand how to do this.
As a pratical example, lets say I have soap dispenser id 1 with a max capacity of 1000ml, I then count 300 pumps, so I know I have 100ml left. I then do a refill and want the number of pumps to get set to 0. Otherwise in the next use it will say I have 97ml available, when in reality I have 997ml because I already made a refill.
Thank you very much in advance.
create table dispenser(
id_dispenser int not null auto_increment,
localization_disp varchar(20) not null,
full_capacity int not null,
primary key (id_dispenser));
create table records(
time_stamp DATETIME DEFAULT CURRENT_TIMESTAMP not null,
dispenser_id int not null,
number_pumps int not null,
battery_level float not null,
primary key (dispenser_id,time_stamp));
create table maintenance(
maintenance_id int not null auto_increment,
maintenance_date DATETIME DEFAULT CURRENT_TIMESTAMP not null,
employee_id int not null,
maintenance_description varchar(20) not null,
dispenser_id int not null,
primary key (maintenance_id));
CREATE VIEW left_capacity
AS
SELECT max(time_stamp) AS calendar,
id_dispenser AS dispenser,
full_capacity AS capacity,
greatest(full_capacity - number_pumps * 3, 0) AS available
FROM records r
INNER JOIN dispenser d
ON d.id_disp = r.id_dispenser
GROUP by id_dispenser;

Answer 1

If I understand correctly you want a view with the amount remaining. This would be the number pumps since the last refill, subject to your formula.
MySQL has had tricky issues with subqueries in views. I think the following is view-safe for MySQL:
select d.*,
(d.full_capacity -
(select count(*) * 3
from records r
where r.id_dispenser = d.id_dispenser and
r.time_stamp > (select max(m.maintenance_date)
from maintenance m
where m.id_dispenser = r.id_dispenser and
m.maintenance_description = 'refill'
)
)
) as available
from dispenser d;