title: how to use trigger in order to increase or decrease stock in mysql
body: <p>I'm creating a MySQL database. I have two different tables: for Sales(<code>id, idoftheproduct, quantity</code>) and for Supplies(<code>id, idoftheproduct, quantity</code>). I want a trigger that whenever we add a new sale or a new supply, it will increase or decrease the values of a different table which is called Stock(<code>idoftheproduct, quantity</code>)</p>