MySQL UPDATE a column value with add or subtract
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 10 in stock, somebody has just bought 5 so I need to subtract that value from the database quantity.
UPDATE products_table SET quantity = quantity - 5 WHERE product_id = 21
Later in the shop admin you receive new stock and so need to UPDATE the product quantity by adding 5 new items to its current quantity of 5.
UPDATE products_table SET quantity = quantity + 5 WHERE product_id = 21