title: how to query the result of UNION query
body: <p>Thank you for your time and apologies if question is silly. I have a result of union query that looks like below. As you can see A and C both have value2 other then NULL. How I can select  B only? I would provide my sql tries but I run out of ideas. I tried count() and sum() and other methods but I always select A B C while I want to get only B.
  Thank you.</p>
  
  <pre><code>value1 value2 
    A    NULL 
    A    1
    B    NULL
    C    2
    C    3
    C    NULL
  </code></pre>
  
  <p>Thank you so much All for your help. I got the answer and much more! You all are awesome!</p>