title: How to set the value of a column based on other column
body: <p>I have the following schema: </p>
  
  <pre><code>customer(cid,cname), 
  books(isbn,price) 
  orders(oid,cid,isbn,odate,quantity,value)
  
  1. A customer can purchase any no.of books on a single order on the same date
  2. A customer can purchase any no. of different types on a single orders
  3. A customer can place multiple orders on a same day
  </code></pre>
  
  <p>So, when a customer place order for isbn say 121,141 whose prices are 500,500 respectively, of quantity 2,5 respectively, the column 'value' must be filled with 1*500=500 and 5*500=2500 respectively. 
  So the value column must be quantity*price always. How can I specify this during the creation of orders table? Should I need a trigger?</p>