title: Without subqueries, SELECT all author_id's for authors with only empty posts
body: <p>Given the following table:</p>
  
  <pre><code>+----+-----------+---------+
  | id | author_id | content |
  +----+-----------+---------+
  | 1  | 1         |         |
  +----+-----------+---------+
  | 2  | 2         |         |
  +----+-----------+---------+
  | 3  | 1         |         |
  +----+-----------+---------+
  | 4  | 2         | XYZ     |
  +----+-----------+---------+
  </code></pre>
  
  <p>How would I select a list of <code>author_id</code>s who have only posts with no content, without using subqueries? In this case, I would want a query that would just return <code>author_id</code> 1.</p>
  
  <p>This works, but I'm wondering if there's a way without having to use sub-queries for the sake of performance.</p>
  
  <pre><code>SELECT author_id
  FROM my_table
  WHERE author_id NOT IN (
      SELECT author_id FROM my_table WHERE content != ''
  )
  GROUP BY author_id
  </code></pre>
  
  <p>Here's <a href="https://www.db-fiddle.com/f/iZ3tMZZZW7MfdVbVseTSSx/0" rel="nofollow noreferrer">a fiddle</a>.</p>