title: Database Application - Store or compute on-the-fly?
body: <p>I have a table of purchase list with fields: ItemName, Quantity, UnitPrice, Amount. Note that Amount is equal to Quantity * UnitPrice.</p>
  
  <p>My simple problem is, should I STORE the amount or COMPUTE it when retrieving the data? To what should I concern about, storage or processing?</p>