title: SQL UPDATE, but only if the old value is null
body: <p>I have been using a sql like this to update a list of properties in my database:</p>
  
  <pre><code>update my_table set a = ?, b = ?, c = ?, d = ?,  where customer = ?
  </code></pre>
  
  <p>But I want to update a property with a new value ONLY if it does not have a value in the database already. How can I do that?</p>