title: How to make "column3" = "column1" - "column2" in SQL
body: <p>In my sql table I have the three columns, price, costs, revenue.  I would like revenue to automatically equal price - costs  (for each separate row, not in total).  And I also would like to know how to do this all at once for existing rows.</p>
  
  <p>Edit: I am using mysql</p>