title: How do I combine columns into one, and filter out NULLs?
body: <p>Here is my table. A list of ids with signup dates in columns newsletter, report, infographics.</p>
  <p><a href="https://i.stack.imgur.com/j1Yr2.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/j1Yr2.jpg" alt="enter image description here" /></a></p>
  <p>I want to combine all those columns into one, without the NULLs</p>
  <p>I've tried the following code</p>
  <pre><code>SELECT id, combined_column
  FROM (
      SELECT id, CONCAT(newsletter, report, infographics) AS combined_column 
      FROM table
  )
  WHERE combined_column IS NOT NULL
  </code></pre>
  <p>But this just gives me a blank table. Is there a way to solve this? Thanks</p>