 <p>As a simple example, let's say you have two columns, A and B.</p> <pre><code>A B 1 100 2 100 3 100 </code></pre> <p>There are three distinct A values, but only one distinct B value. It would be impossible for <code>COUNT(DISTINCT *)</code> to return a single, meaningful value. That is why that syntax cannot work.</p>