title: Insert (if not present) and return id
body: <p>I have a unique column. I want to insert a row if it's not already there, and then return the <code>id</code> of that row.</p>
  
  <pre><code>INSERT INTO t(a) VALUES ('a') ON CONFLICT DO NOTHING RETURNING t.id;
  </code></pre>
  
  <p>returns nothing at all. <a href="http://sqlfiddle.com/#!17/a7201/6" rel="nofollow noreferrer">Here's a fiddle</a>.<br>
  I'm looking for how to get 1 each time, whether 'a' was newly inserted or not.</p>