title: SQL to select rows where all associations have not null values
body: <p>I have two tables: <code>workflows</code> and <code>tasks</code>. Tasks belong to a workflow.</p>
  <p>I want to select all workflows where <strong>all</strong> the tasks associated with them are complete.</p>
  <p>For example:</p>
  <p>Workflow 1</p>
  <ul>
  <li>Task A - <code>completed_at = 2020-01-01</code></li>
  <li>Task B - <code>completed_at = null</code></li>
  </ul>
  <p>Workflow 2</p>
  <ul>
  <li>Task C - <code>completed_at = 2020-01-01</code></li>
  <li>Task D - <code>completed_at = 2020-01-01</code></li>
  </ul>
  <p>Workflow 3</p>
  <ul>
  <li>Task E - <code>completed_at = 2020-01-01</code></li>
  <li>Task F - <code>completed_at = 2020-01-01</code></li>
  </ul>
  <p>I want to run a query that only selects workflow 2 and 3 since their tasks are complete and have no incomplete tasks.</p>