title: Conditional Value
body: <p>I have a table that uses UUIDs as primary keys. New rows are inserted like this</p>
  
  <pre><code>INSERT INTO a ( id, ... ) VALUES ( uuid_generate_v4(), ...)
  </code></pre>
  
  <p>Now I actually only want to generate the UUID when no ID is provided in the insert (either <code>NULL</code> or an empty string)</p>
  
  <p>Is it possible to write something like this?</p>
  
  <pre><code>INSERT INTO a ( id, ... ) VALUES ( $1 || uuid_generate_v4(), ...)
  </code></pre>