title: Sql: select another field if one field is blank or 0
body: <p>I have a table Product where i have fields id,name,selling_price and special_selling_price.
  I want to fetch "special_selling_price" of all records BUT if</p>
  
  <pre><code>`special_selling_price`=0 OR `special_selling_price`=null
  </code></pre>
  
  <p>then it should fetch "selling_price".</p>
  
  <p>If my records are</p>
  
  <pre><code>id    name    selling_price    special_selling_price
   1      A           100                   75
   2      B           200                   0
   3      C           300
   4      D           400                  150
  </code></pre>
  
  <p>Then it should fetch</p>
  
  <pre><code>75
  200
  300
  150
  </code></pre>