title: Doing math while updating Postgresql database
body: <p>I am working on a small invoicing solution and I Need to add a column to store the price of one unit. I already have a column for total for all the units and the quantity in the db. </p>
  
  <p>My question is, how can I add this column and populate it with accurate numbers? I know that formula will be:</p>
  
  <p><code>total_col / quantity_col = unit_col</code></p>