title: update column only when it is null else dont update
body: <p>Is there a way to update a column only when it is <code>null</code> and leave it as it is when it is not <code>null</code> in one multiple-columns-update query?
  something like below. (like how we use case in select statements)</p>
  
  <pre><code>UPDATE users  SET users.city = 'Dallas',
        CASE
            WHEN users.Global_id IS NULL
            THEN
                users.Global_id = '123'
        END WHERE userid = '12312312'
  </code></pre>
  
  <p>The above update statement throws <em>ORA-00927: missing equal sign</em> error.</p>
  
  <p>Reason why I am looking for this? 
  I have a schedule job that runs a similar query like above. 
  And there also exists a trigger in <code>users</code> table that will raise error if you try to update a 'not null' <code>global_id</code>. So my job fails when it encounters this trigger.</p>
  
  <p>I have one option... to split this update query to two.. one to update city and one to update <code>global_id</code> where <code>global_id is null</code>.</p>
  
  <p>But wondering if this can be achieved by any other way... Any idea would be appreciated.</p>