title: When to use ?? and ? as placeholders in node-mysql for building a query?
body: <p>I am looking at some examples in node-mysql
  <a href="https://github.com/felixge/node-mysql" rel="nofollow">https://github.com/felixge/node-mysql</a></p>
  
  <p>I am confused over when to use ?? and ? as placeholders in constructing a query.</p>
  
  <p>An example is here;</p>
  
  <pre><code>var userId = 1;
  var columns = ['username', 'email'];
  var query = connection.query('SELECT ?? FROM ?? WHERE id = ?', [columns, 'users', userId], function(err, results) {
    // ...
  });
  
  console.log(query.sql); // SELECT `username`, `email` FROM `users` WHERE id = 1
  </code></pre>
  
  <p>How do I know when to use <code>??</code> and when to use <code>?</code> ?</p>