title: Check if internal (nested) MySQL query returned a value
body: <p>I have a query:</p>
  
  <pre><code>SELECT 1 FROM block_list WHERE client=123
  </code></pre>
  
  <p>which is nested within</p>
  
  <pre><code>SELECT * FROM products
  WHERE (client=123 AND product=234)
    AND 1 &lt;&gt; (SELECT 1 FROM block_list WHERE client=123)
  </code></pre>
  
  <p>It works (other way round) if I replace <code>&lt;&gt;</code> with <code>=</code>. But I want it to return NO records if the user is in the blocked list. Block is temporary and affects only some parts of the website.</p>
  
  <p>Is tehre a way to check if nested query returned <strong>no results</strong> within WHERE clause?</p>