title: Fastest way to select a record from another table if doesn't exist in the first table
body: <p>Perhaps this is something very easy for SQL experts.</p>
  
  <p>I have two tables:</p>
  
  <pre><code>Table A     TID    Desc
  Table B     TID    Desc
  </code></pre>
  
  <p>I understand I can do <code>IsExists</code> but not sure if that is the fastest way.  The requirement is that if the record exists in Table A for a given TID, the record should be read form Table A, else from table B.  Record will definitely exist in Table B</p>