title: How to "prefer" data from one table over another in SQL, help reformulating.
body: <p>Using Sql management Studio on Sql-Server 2005.</p>
  
  <p>I guess my title is not very clear, so although I'd appreciate a direct answer, even better would be help in reformulating the question that I can search for an answer by myself and learn in the process. </p>
  
  <p>Table A contains yearly total sales for various products. This table is updated once a year in Feb/March.There are two consequences to that:  A product that started being sold in, say, July, will obviously only have a six month sale total for the year. Also, any new product will not appear in it before the following year. </p>
  
  <p>Table B is an exception table that gives an estimate for all products not yet in table A or that have a less than a twelve months sale history. </p>
  
  <p>Table C joins the yearly sales with various info from other tables and also contains calculated columns .</p>
  
  <p>In pseudo code what I need is:</p>
  
  <blockquote>
    <p>Use sales of product x from Table A in table C unless sales for this product exist in table B.</p>
  </blockquote>
  
  <p>Basically, data from table B should take precedence over table A wether the record exists in A or not. </p>