If you have a numerical value in a MySQL database table, that you want to UPDATE by adding a value to it, or subtracting a value from it. This can be done with a single MySQL query.
So for an example, I have a database with a product which has a quantity of 100 in stock, somebody has just bought 10 so I need to subtract that value from the database quantity.
1. UPDATE products_table SET quantity = quantity - 10 WHERE product_id = 1001
Later in the shop admin you receive new stock and so need to UPDATE the product quantity by adding 10 new items to its current quantity of 5.
2. UPDATE products_table SET quantity = quantity + 10 WHERE product_id = 1001
