如何将我的POS表中的数量减去产品表中的数量

Question

example. i have product table which have a quantity etc.

item name: Shoes.
Quantity: 20

POS table. If the user bought the bag which have a quantity 20.

Itenm name: Shoes
Quantity: 5

the question is. how can i subtract the quantity : 5 in POS table to the quantity 20 in product table. thanks.

Answer 1

You could join the two tables on item name and do the subtraction.

select p.item_name,
    p.quantity - po.quantity
from product p
join POS po on p.item_name = po.item_name;

If you need to update the product table with the difference, then:

update product p
join POS po on p.item_name = po.item_name
set p.quantity = p.quantity - po.quantity;

If there are multiple rows with same item name, you probably want to aggregation them first and then join to update:

update product p
join (
    select item_name,
        sum(quantity) as quantity
    from POS
    group by item_name
    ) po on p.item_name = po.item_name
set p.quantity = p.quantity - po.quantity;