By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table Ingredients (ing_id int not null,
ing_name varchar(40), preserve_time int, ing_bought date, freshness int,
primary key (ing_id));
insert into Ingredients values (1, "Meat", 18, "2020-10-20", null);
select * from Ingredients
| ing_id | ing_name | preserve_time | ing_bought | freshness |
|---|---|---|---|---|
| 1 | Meat | 18 | 2020-10-20 | null |
update Ingredients set freshness = 100 - (DATEDIFF(now(), ing_bought) / preserve_time) * 100
where ing_id = 1;
select *, DATE(NOW()) as today, DATEDIFF(now(), ing_bought) as days from Ingredients
| ing_id | ing_name | preserve_time | ing_bought | freshness | today | days |
|---|---|---|---|---|---|---|
| 1 | Meat | 18 | 2020-10-20 | 6 | 2020-11-06 | 17 |