title: if conditions in where clause
body: <p>I want to retrieve the columns from the table., depending on the conditions.,
  what I can use.</p>
  
  <p>For example, I' ve fields namely, add order comments, cancel order comments, postpone order comments, action(add, cancel, postpone) and amount received(Y/N)</p>
  
  <p>Now I've to fetch columns add order comments, cancel order comments, postpone order comments depending on action and amount received.,</p>
  
  <pre><code>if(action='add' and amount received='Y')
  then
  i've to fetch add order comments column
  elseif(action='postpone' and amount received='Y')
  then
  i've to fetch postpone order comments column
  else (action='cancel')
  then i've to fetch cancel order comments
  </code></pre>
  
  <p>how to get this done in sql or plsql., I want this conditions in select statement</p>