title: What does "expr" do in MySQL's COUNT(expr) function?
body: <p>Consider below:</p>
  
  <pre><code>SELECT COUNT(*) FROM table;
  SELECT COUNT(1) FROM table;
  SELECT COUNT(-2) FROM table;
  SELECT COUNT(135392) FROM table;
  SELECT COUNT(field) FROM table;
  SELECT COUNT(field1 + field2) FROM table;
  </code></pre>
  
  <p>I am not clear on what <code>expr</code> actually does, or what it can be used for because all of the above SQL statements return the same result.  Example below:</p>
  
  <pre><code>+-----------+
  | count(..) |
  +-----------+
  |     54542 |
  +-----------+
  </code></pre>
  
  <p>MySQL's manual (<a href="https://dev.mysql.com/doc/refman/8.0/en/counting-rows.html" rel="nofollow noreferrer">https://dev.mysql.com/doc/refman/8.0/en/counting-rows.html</a>) does not go into very much detail on the <code>expr</code> part, other than using the <code>*</code> symbol</p>