title: What is the best way to stock products with different purchase prices & expiration date?
body: <p>I'm developing a stock inventory system using PHP, Mysql. This system have purchase module to add/update/increase quantity and expire date of products &amp; sale module to decrease/out the quantity of products.</p>
  
  <p>Now what is creating confusion to me and what is my real question is that </p>
  
  <ul>
  <li>Suppose we have PRODUCT A1 in our stock with quantity 20 and the price of
  PRODUCT A1 was $15 when we purchase it.</li>
  <li>Now we want to stock this product more for example we want to buy 100 more items of PRODUCT A1 but now new price of PRODUCT A1 is $18.</li>
  </ul>
  
  <p>What is best way/logic to make these items separate to know how much products are purchased with price $15 and new price $18 on sale page.</p>
  
  <ol>
  <li><p>Should I have to store 2 or more quantity column in database?</p></li>
  <li><p>Should I have to store same product more then once in my database?</p></li>
  </ol>
  
  <blockquote>
    <p>Currently I'm generating average of $15 and $18 then adding this value
    as purchase price of product.</p>
  </blockquote>