title: Insert new row for binary systems with SQL
body: <p>I have the following table</p>
  
  <pre><code> (cl1  ,  cl2)
    ----  ----
    (a   ,  1)
    (a   ,  2)
    (b   ,  2)
    (c   ,  1)
    (c   ,  2)
  </code></pre>
  
  <p>each a , b ,c  can take two values (1 or 2 or both). </p>
  
  <p>My question is :<br>
  How to insert a new row (with 0 on cl2) for all the cl1 that have only 1 or 2 and NOT the both in the example. I would like to insert the following row :</p>
  
  <pre><code>----
  (b   ,  0)
  ----
  </code></pre>