title: turn null values when update
body: <p>i have two tables. sales and produclist. some of the data existing in both table are common. so what i want to do is that i want to update selected fields in tbl productlist  where tbl prodctlist data exist in tbl sales. i used this following query below but the problem is that  if data from produclist is not existing in sales it turns to null. what i am looking for is that if data from tbl productlist does not exist in tbl sales it will not turn to null, instead stay the same for what value it has before updating</p>
  
  <pre><code>mysql_query("UPDATE productlist 
  SET pleft=pleft+(SELECT SUM(qty) FROM sales 
  WHERE (sales.name = productlist.pdesc) 
  AND (sales.name1 = productlist.pdesc1))"); 
  </code></pre>