title: MySQL shows no result when empty pivottable in query
body: <p>This query in MySQL 5.7.28 Ubuntu:</p>
  
  <pre><code>SELECT
      p.*
  FROM
      testvvs as p,
      testvvs_privs AS priv
  WHERE
      p.parent = 0 OR (p.userid = priv.userid AND priv.writepriv = 1)
  </code></pre>
  
  <p>gets an empty result if pivottable priv is empty, correct result when <strong>any</strong> arbitrary value in priv. What's going on?</p>
  
  <p>Tanks and kind regards</p>