title: Oracle concatenate if the column doesn't already contain value
body: <p>I have the following Oracle code:</p>
  
  <pre><code>update registration set 
      reg_comments=pMessage||decode(reg_reg_int_hld_wait,1,'Was on hold ')||reg_comments
  where reg_rid=v_reg_rid;
  </code></pre>
  
  <p>I want to only concatenate the reg_comments to itself IF it doesn't already contain pMessage.</p>
  
  <p>How would I do that?</p>