title: How to make a select statement return null if no value is returned?
body: <p>Let's say we have the table "letters" like so:</p>
  
  <pre><code>a | b  
  -----  
  0 | 0  
  0 | 1  
  1 | 0  
  1 | 1
  </code></pre>
  
  <p>And the following select statement:</p>
  
  <pre><code>SELECT val1, val2, x.a. val3
    FROM table1, 
         table2, 
         (SELECT a 
            FROM letters 
           WHERE a=b 
             AND a=0) x, 
         table3
   WHERE table1.val1 = table2.val1
     AND table1.val2 = table3.val3
  </code></pre>
  
  <p>I need the embeded SELECT statement (x) to return:</p>
  
  <pre><code>0  
  NULL  
  NULL  
  NULL 
  </code></pre>
  
  <p>Instead of what is currently returning which is </p>
  
  <pre><code>0  
  </code></pre>
  
  <p>Then I want to be able to use that in the big SELECT statement.
  I hope this makes sense!</p>